going with a smaller diameter rim will help with performance aswell, 17" is best but 18" isn't too harmful either.

you should also probably consider how loud you want it to be? once you know that you can decide on the number of resonators you'll get made and the type of muffler.

if you want something quieter 2 resonators and an oval shape (i.e. non-cannon) muffler will probably work better

otherwise one resonator and a canon style muffler should be a good compromise.

the tough thing with a custom system is that there's no guarantee on sound quality/volume.. most pre-made exhausts seem to have a good balance between sound/performance.