Your existing gears are actually 3.07's. (43/14= 3.07)

3:77 would make a "great " improvement but be prepared for way more RPM's on the highway & fuel mileage drop BUT way more fun to drive. 3:55's would also help & a little more "friendly"!! I went from 3;07 [3:08] to 3:91 in impala and now revs 3200 at 50 MPH & over 4000 at 60 but sure gets to those speeds pretty quick!!!

Was yours assembled in Australia or fully imported? The 1967-68 Impalas and Parisiennes were all 4BBL low comp 327 (250hp 9.0:1 L73 code in 1968, just a COPO engine in 1967 240hp 8.75:1) with Powerglide and 3.55 12 bolt rear axle, not LSD/posi. The 1965's were either a 195hp 9.25:1 2BBL 283 or a 230hp 8.75:1 4BBL 327 (COPO again). Both Powerglide with open 3.55. 1966 were basically the same as 1965 driveline wise.

They were Tonawanda engines as McKinnon didn't do 4BBL engines until later in 1969. The 250hp L73 was a standard engine in US Chevrolets for 1968 but the 230/240hp versions in 1966 and 1967 were COPO only, often only Police cars.
GMH would have wanted a 4BBL engine to parallel what Ford were offering over here.

Interesting correlation between the 1966-67-68 engines. Apart from different dress across the year model engines where various bits moved or were added along the way:

1966 230hp. Straight flat top 327, GP hydraulic cam and 75cc heads (8.55:1 compression, rounded up to 8.75), Rochester 4GC.
1967 240hp. Identical engine but Quadrajet fitted, good for 10hp.
1968 250hp (L73). Heads changed to new 307 70cc heads for just over 9:1 compression but otherwise the same as 1967 (although clean air stuff added in USA), another 10hp rise due to the bump in compression. It is interesting that no GM or GMH literature quotes 9:1 for this engine, they leave it at 8.75:1 yet give it 10hp extra. They do acknowledge the smaller combustion chamber though in the Engineering Technical Specifications. Funnily enough, the 307 with 1/8" smaller bore, flat tops, same stroke and same heads is quoted as 8.75:1, which is impossible for them both to be the same compression ratio. The two 327's in 1969 (210hp small 2BBL in Camaro and 235hp large 2BBL in full size) do quote as 9:1 and have the same 70cc heads as the 1968 L73, albeit the 1969 versions with accessory holes.

We got the 250hp engine here in Australia in the HK Monaro GTS327 as well as in the1968 Impala and Parisienne. Coupled to a Saginaw 4spd and 10 bolt 3.36 posi rear axle (with 14" tyres so roughly equivalent to 3.73 with 15" tyres). In proper tune was good for 15.4s quarters and could pull 135mph. Bit of rough journalism here about the car, but reasonably factual:
www.whichcar.com.au/features/holdens-pre-commodore-heroes-hk-monaro-gts-327

-- Edited by HK1837 on Tuesday 1st of September 2020 02:19:58 AM

-- Edited by HK1837 on Tuesday 1st of September 2020 02:21:58 AM

But were the intakes actually McKinnon cast? The info I have is that McKinnon didn't cast 4 barrel intakes (and 350 engines) until 1969, and it tooled for 327 in 1966 and 307 in 1967. In fact the 4BBL 327's we got here in 1966-1967 here appear to be all McKinnon assembled (and presumably machined as well) engines but the blocks and intakes were cast in Saginaw and/or Tonawanda. Whereas all the 283 and 307 here were 100% McKinnon, all castings. Even the 4BBL 327's built for the final GTS327's in Australia in GM 1969 model year (in the final months of 1968) had some Tonawanda parts, the block and heads were McKinnon cast (3932386 block and 3927188 heads) and the whole thing machined and assembled at McKinnon, but the 4BBL intake (3927184) and one of the two exhaust manifolds were Tonawanda cast - the reason being it was another 6 months until McKinnon were casting 4BBL intakes (as far as I know).

 

It may just be Quadrajet 4BBL intakes too that they didn't cast until 1969? It just seems very odd that all 2BBL engines in the Aussie cars that were otherwise all Canadian were McKinnon, but as soon as it was a 4BBL engine it was either a McKinnon engine using some Tonawanda/Saginaw cast bits or whole Tonawanda engines. Even the L48 and LM1 engines that were fitted to HT GTS350 Monaro, were all made in Tonawanda in the early months of 1969, prior to McKinnon tooling for 350 and 4BBL. The final L48 engines for the last of the HG GTS350 in late 1970 were full McKinnon cast and assembled with 3870010 blocks, 3973370 heads and 3973469 intakes. All HQ GTS350 L48 engines (1971-1974) were McKinnon cast and assembled.

 

-- Edited by HK1837 on Tuesday 1st of September 2020 05:33:43 PM

koolconvertible wrote:

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HK1837 wrote:

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Was yours assembled in Australia or fully imported? The 1967-68 Impalas and Parisiennes were all 4BBL low comp 327 (250hp 9.0:1 L73 code in 1968, just a COPO engine in 1967 240hp 8.75:1) with Powerglide and 3.55 12 bolt rear axle, not LSD/posi. The 1965's were either a 195hp 9.25:1 2BBL 283 or a 230hp 8.75:1 4BBL 327 (COPO again). Both Powerglide with open 3.55. 1966 were basically the same as 1965 driveline wise.

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 Only our thin pillar sedans and four door pillarless were assembled in australia from CKD kits. Other variants, like my convertible, could be ordered through the GMH dealership network but were fully assembled in canada for the RHD market. The reason why I can access production details through vintage vehicle services Canada.

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Ah, yours is one of those cars. Southern Motors (Bob Jane) was a dealer of them, I think it was this access to GM in North America that allowed him to get hold of two of the 1969 ZL1 Camaros, one of which he won the ATCC in 1971 and 1972. There was a full list of the cars Southern Motors sold on Ebay a few years back. A friend of mine has the Le Mans that was on the brochure that Southern Motors did out on a pier somewhere. I can't remember if I kept copied that document or not.