Consider what halftank says, that's where you really have to watch out:
Whether you pimp your 265 or throw in a 350, it's going to have a lot more power, one way or the other. If you old powerglide isn't up to par, it'll brake in the first 500 mls. That is, if you don't hit a tree because the brakes can't handle your "new" speed anymore. And your shocks and springs might sag on one side if you put in an engine with a lot of torque. You need to see FIRST exactly what your aiming at, THEN see what things other than engine-stuff you need to make the car fun. Once you start buying and building, don't leave your chosen path too far.
And I agree with the rest on keeping the 265 in a couple of garbage bags in a dry place with loads of WD40 covering everything (spray it in the intake, the exhaust-ports and literally everywhere you can get to, if it sits for 5 years now you'll want to rebuild it anyway).
Then get a 350 and start playing
Maybe pull the powerglide and throw in a 350 or 400...
That's just me thinking loud...
Greetings from across the pond,
Mad