What size exhaust should I use?

so sonds like a 350 30 over an towing cam mabe 202 heads . so about 300 horse power thre inch look cool but 2/ half will sond better an give a little back presser you need an . headers or iorn manfoulds . no y or h just straightt back an one set of cherry bboms or a hus trust mulfers will sond good . had same motor in a 1969 c 10 miss that truck