Overdrive transmission

I'm in the same boat in that I've got a small block with a 727 and I'd like to upgrade to something with an extra gear; but with the caveat that I'd prefer something that can bolt up to a G3 hemi down the line.

I'm not going to be making beaucoup power so the 500 & 42 look enticing in that they're smaller, lighter and don't rob as much HP, and I'm guessing less cutting? But I have no problem going for the beef of the 518 or the 46 and honestly wouldn't mind going 2004R if that's the right fit.

money isn't necessarily a concern, but I'm not looking to break the bank or anything. the car does get driven in a spirited manner and I'm prone to road tripping it. dealer's choice on gears, but I've currently got 2.23's in it.

what's the consensus cost wise, ease of install, performance and future compatibility?
Here's what I had to cut on the crossmember to fit the 200-4R in my '73 Sport. That and buying the adapter kit was the only big issue with going with this trans. No cutting on the tunnel is required.

Crossmember passenger cut.jpg