hmmm 48trim .63a/r, what did those come on originally?

with all of these small OEM turbos, to a certain extent running more boost will put the compressor in a better place as they have pretty narrow upswept maps. For example if you tried 7psi and 6000rpm you'd probably find yourself well off to the right of the map.

That doesn't say anything about the exhaust side.

Based on my own experience I wouldn't want anything smaller than the 60 trim .63a/r that I used on my 383 on a 360 using external wastegates. I've kept those turbos because I've got a 318 in a 72 RR clone that I think they would also be pretty good on.

If I were you I would still run a front mount single, maybe a big T4 would get you by and then you don't have to struggle with the silly talk that a T6 must have 3' of 5" straight off the exhaust, which gets more than a little difficult to package.