2 1/4 exhaust slant 6

Gasses expand as they cool, the temp of the gasses will only drop the farther they get from the engine, so..

I can't tell you what exhaust size is best for your setup but no, that won't make it ineffective. If you want to make the most of it, make sure you build the exhaust with mandrel bends and not some crushed and kinked muffler shop piping.