has anyone used a diff AC Comp on the stock AC unit-looking at a setup that uses the old "pancake" comp, but wanted to keep my stock firewall/dash componets

I used an Omega 20-10206 comp. pt #AAA 06-03002N looks like a sanden 508, but all the factory lines bolt on the rear & will bolt right to the factory brackets like the old A6 they look very nice & work very well have one on my 70 & 71 hope this helps. Jim

Don't bother with an R4 (pancake) compressor, they're just not very reliable. Replacing the A6 with an R4 would be a serious downgrade to your system.
The comp. that Jim mentioned sounds like a Pro6Ten, which is a very good swap. There's no real performance difference, but they look nicer than the old A6 and they're a good bit lighter.
You won't find a modern compressor that performs "better" than the A6, though. All modern compressors have less displacement, so in theory they don't work "as good". In operation, you won't notice the difference unless you're in the desert, in mid summer, at high noon.

Don't bother with an R4 (pancake) compressor, they're just not very reliable. Replacing the A6 with an R4 would be a serious downgrade to your system.
The comp. that Jim mentioned sounds like a Pro6Ten, which is a very good swap. There's no real performance difference, but they look nicer than the old A6 and they're a good bit lighter.
You won't find a modern compressor that performs "better" than the A6, though. All modern compressors have less displacement, so in theory they don't work "as good". In operation, you won't notice the difference unless you're in the desert, in mid summer, at high noon.

Edit: I just looked at the Omega compressor. It's not a Pro6Ten, but it's the same idea. Sanden 508 internals and it will bolt to your old brackets.