sorry to see that you got in trouble with your cad. I hade the same issue than you with my 77 fleetwood 4 years ago, and more recently with my 78 fleetwood fuel injection. Exactlty the same things: good idling, and bucking at high speed. With my 77 carb, there were no evidence of backfiring. just a gazoline smell int he exhaust system, just like the engine was running a little bit too rich. With my fuel injected 78, I had lots of bacfiring. I solve the problem when I replaced the vacuum timing advance on the distributor, and the pickup coil. Have you check these part? of course you need to open the distributor, and remove quite everything inside to get the pickup coil and the timing advance system. I have the shop manual, so if you need any other info, just let me know !
Good luck. Jeff.