Why not rebuild the 302 that you have on the engine stand
If that is the correct engine for your car and you have no plans to all out race the motor, just rebuilding that should be fine.
If you are keeping the engine for origiinality and just going to build the 400 to save the 302 for some future rebuild, then
The 400 small block Chevy is a good engine, if built correctly. In my opinion if you are buying new rods and pistons for the
400, I would switch it to 5.7 rods clearanced for the 400 stroke. There should not be a lot of additional cost for that.
The is no substitute for cubic inches, but then there is no substitute for the quickness and high rpms of a 302 either.