[DRJDVM's '69]

Personally I'd go with the 2 separate mufflers rather than the big crossflow...... Better flow and better sound

As for the pipe.... Mandrel bent tube is better since it won't have kinks or wrinkles at the bends. General rule is 2.5 inch pipe will support up to around 500hp no problem.... When you start pushing much over that it's best to go up to 3 inch

If you do a search there are tons of discussions on exhaust size