It is always risky offering advice on what fits. Is your 55 year old car 100% stock? A '68+ A-body rear end is an inch wider but bolts in perfectly. Have the rear quarters been changed? I've seen new quarters put on over old ones so the fender lip is almost 2 inches wide! Stock spindles? Anyone replace the springs, or use a torch to heat a spring or cut off a coil for a cheap lowering kit? There are many different options for disk brakes on the front. Some move the wheel outboard by more than 1/2". You say "Factory disk brakes up front". I thought disk brakes weren't available until '67...

You should be able to fit 18 x 8's with zero offset (4.5" BS) on the rear. From my experience, zero offset is typically the right choice, for 60's A-bodies, but all that's with a completely stock set up. 18x9's should fit too but I've never actually put a set on. I have fit 15x10's with zero offset on a '66 GTO but I had to trim the lower shock mount. Same problem on Chevelles (and Beaumont). Also, I did get some rubbing on the inside inner fender when one wheel was up on a curb. But again, that's with a stock (pre-'68) rear end. A 16 inch or larger rear wheel should clear the shock mount but you might still rub on the inner or outer fender. I knew one guy who had 315's (315/40 15's if I remember correctly) on the rear of his Chevelle and claimed no modifications except a little massaging of the fender lip (using a baseball bat). I am not sure what his wheels were or what offset. Numerous examples of 275's and even 295's out there, but the 315's were amazing.

Up front, I know many people claim 15x8's up front with up to 245 width. I know for certain 15 x 7's with 225's fit. Zero offset worked best for me, but again, that assumes stock spindles, springs and brakes.

I am a strong believer that if you really want to fill the wheel wells, you have to measure up YOUR car. Even if you know for certain everything is 100% stock, weak body mounts can allow the whole body to shift, springs sag over time etc. And make sure the stance is where you want it before you settle on the wheels & tires.. Measuring up, buying wheels & tires then changing spindles, springs, control arms, brakes etc can lead to problems.

The easiest way to measure up is to get a set of wheels & tires close to what you want to run and mount them on the car, then get underneath and start measuring. If you can only get 18x7's you can mount those, take measurements and figure out how much clearance you'll have with wider wheels/tires. Even working from what you have on it now will allow you to get the measurements you need. Just measure, sketch it up and then figure out what the wider/bigger wheel & tire is going to do.

As a caution (and some personal opinion), be aware that the suspension on these old cars really don't like big wheels. Yes you can run 18's or even larger, but they originally came with 14's and huge sidewalls. The lack of sidewall produces a jarring ride. It's amazing how well modern suspensions absorb that impact.

Sorry for the long winded reply.