Don's will get you a very close reproduction. They might be at Carlisle right now.
No X.
I'm not 100% positive for 1970 but there was no H on 340s in 1968 and 69. Look in the shop manual.
Differences I'm aware of from the pre 70 cars that might be releavant to your 70.
Accurate's pipes use the correct muffler hangers, but last I checked, not the correct tail pipe hangers.
TTI uses the correct tail pipe hanger but hangs the mufflers their own way.
Accurate's headpipe gasket is a nice repop with a fire ring.