5.0mustang5speed

my brother is going to injection.i can buy his 850 mighty demon for cheap and it has less than 20 miles on it, so its brand new ...is it to big for my motor..347 ford, worked proline heads 2.02 valves 200cc runners, 10.9-1 comp...619 sloid roller cam..he has a motor that is about the same as mine but is a 355 chevy...and we pulled the plugs and they are black as black can be..which is rich...now can i jet the 850 down enough for my motor...