X pipe is supposed to be better, but from experiences, the "H" pipe is quieter.
IF you find a crack in an exhaust manifold, I wonder if new stock ones are available...?surely yes...
www.cliffshighperformance.com is THE place to get your carb kit and parts. Deluxe kit is $80.00 (worth every penny!) but has parts that nobody else includes. Need to contact by email at first. Tell him your exact engine specs, purpose for vehicle and carb number. Cliff can send you the EXACT parts that you will need, such as jets and metering rods. Todays gasoline is a different deal from even 10 years ago, tune is different. AND his parts are ethanol resistant!