Backfire: Usually because of too late timing, but be sure to check your Total mechanical advance (4000 rpm, vacuum disconnected and plugged) is not above 36 degrees, if it is, don't just advance the timing anymore, or it could be expensive. Let me know if it is, and I'll try to set up your ignition advance curve from remote. And yes a backfire can destroy a powervalve, in fact the old Holleys usually destroyed the powervalve on the first backfire, I had a Dominator that did that. But modern Holleys (and my preferred carb: Summit) are protected by one-way valve systems with a check ball, so it shouldn't happen.
Pop in the exhaust: Yes, too much fuel indication a rich mixture, could be powervalve, could be idle circuit, could be primary jets, could even be float level. My previous Pantera did it a lot, I liked the sound, and never did anything to get rid of it
Clutch, I know nothing about...