May or may not help but there is only 5/8 '' between the top of the cap and the bonnet on my 60 and that is not a powerglide radiator. If yours had the wrong core last time perhaps someone with an original radiator could measure their core for you.

You should measure the distance between the top bolt where the fan shroud and rad bolt on and the top of the core. I have a 62 409 reproduction core in my 63, plus 63's and 64's have a different bolt pattern on the rads, or at least the fan shrouds have a different bolt pattern.

The 283's had a smaller rad, but you could get a 409 in a 64, so I'd guess you'd have room for 5/8" more height.

FWIW, my top of bolt to top of rad in 3.5 inches; top of rad to bottom inside of hood is over 2 inches (measured with playdough). My top of rad sets about 1/2 inch at most above the top of the rad support.