I am building a motor but in the mean time i had a 502 mpi in my boat and the block is no good. I do have a 454 block that is good.... could I bolt my 502 top end and cam on that block and make it work for now??

What is wrong with the block?
To switch things over it depends on what version you are dealing with Mark IV, Gen V or Gen VI.

It is a gen V

A bit rich is likely the issue. I ran 502 efi's on my 454's but they could handle the rxtra fuel. Mine was a 96 efi on a gen 4.