If it were me...I'd go 2 1/4" exhaust with that motor. I personally think 3" would be too big. Reason for that is, low volumes of exhaust flow faster through smaller tubes, therefore scavenging the cylinders more completely. The smaller exhaust obviously will not flow the volume needed at high rpm, but it sounds like the OP isn't really ever going to visit the high rpm neighborhood hardly at all.
Just IMHO, but that's always been my understanding of it, but I'm young and dumb too
