You need to get rid of those 'put a chev in it' people in your life. The 403 already has more torque than a 350 will see, and it's stock and strangled down to suit an emissions role. You can get some very decent power out of one and still appear stock under the hood which can be important with original cars or those that don't allow much in the way of taller intakes or carb swaps. A slightly warm 403 and gear swap will leave most 350 posers behind, and all before the revs could rise.
Secondly, you will have all sorts of dramas getting a 350 swap and not end up bolting the shaker to the hood. It already sounds like you are somewhat proud of the fact you have a genuine model, rather than a Camaro. An engine swap will see the value of the car not even match the engine you just put in.
Rebuilt is probably just that, slight overbore but otherwise stock replacement, certainly not what you will get excited over. But if still in good order you can still double the power with a few clever bolt-ons. Plenty of posts on these already.