If it is a stock 265, boring the cylinders will not contribute enough horsepower to notice UNLESS the cylinders are worn and need to be bored.

The thing has a cast iron powerglide with a relatively tall first gear. You can put a 700R4 (has overdrive) in the car but you need to know what you are doing....have you done anything like this before? The stocker had motor mounts in the front of the engine and mounts on the bell housing. Later model stuff have mounts on the side of the engine and a transmission mount at the rear of the tranny. It is not good to mix the two styles (front mounts and a rear tranny mount) since the stability of the powertrain is lessened. Therefore, if you do a late model trans, you should also go to a later model block with side mounts.

Any small block chebbie V8 engine will bolt in directly. There are two water pump lengths and a few other details but it is pretty straightforward and has been done at least four zillion times. Later model blocks still have the provision for the front mounts so you can go new engine/old transmission if you want......until the old powerglide pukes at least.

BUT....if you are "restoring" the car, any changes past stock mean that you are not restoring it anymore.....you're building a hot rod!!