I can tell you that an X-pipe or an H-pipe will certainly help torque because it essentially connects and extends the header collectors but doesn't slow the velocity of the exhaust gas. Each header is like one 4-cylinder engine--an X or H-pipe helps use all 8 exhaust pulses, which improves scavenging in the low and mid-range, but you lose nothing on top-end. This also tends to quieten the exhaust some, too. You won't see 20ft.lbs, but they are well worth using. From my experience, H-pipes are easier to install and cheaper than X-pipes, and if you flange the exhaust pipes near the mufflers, you can remove the while thing if you ever have to drop the transmission, leaving the tailpiipes intact. I'll be using this on my '68 along with 3" pipes and Warlock mufflers. I don't know anything about the 70-Series Flowmasters or Edelbrocks--hopefully someone else does. Good luck!