Want to swap my 1975 318 for a 1977 360, any problems?

Flex plate or torque converter may need weights to balance, .

Yup. Easiest way is to by a B&M aftermarket flexplate just for this situation. You'll want the one described as neutral balance converter, for 360LA (not magnum) to 727 trans. I believe they are about 89 bucks.

If you ever decide to go back to a 340/ 318, all you need to do is drop in a factory flex plate.