Swap or rebuild?

I should have been more specific, I was wanting to keep the engine cost down and I am not sure if I find a 5.9 that I can see run or use a scope to inspect the pistons would be better than investing into the 318 that runs good but doesn't have enough power. I bought it with a new Edelbrock carb with a performer intake, it also has Hedmans and 2 1/2" exhaust already. I have curved the distributor and tuned the engine, just doesn't have the guts to spin much with a 2.76 rear. I already have brakes and rearend, etc in the works.