1975 Ford F-250 Excellent Idle-Bogs/Backfires under Load

My truck has a 390 w/electronic ignition and runs excellent. Went to start it and noticed it ran at an idle right when started (usually cold starting and you have to keep your foot on the gas for 15 mins before it will idle). Put it in drive and it bogged when I gave it gas-real bad! Reved it up but it would not clear up, acted like it wanted to die, but didn't. Let foot off gas, rpms back up to a perfect idle. Changed wires, fuel filter/pump. Was gunk in the pump filter. Now wondering if it might be the electronic ignition, cap, rotor, or carb needs rebuild? Any ideas folks? Like I said, it runs excellent at an idle. Wouldn't think it had any problems if you heard it run. One other symptom is that when you turn the key over, it seems like it turns over forever and won't fire up but as soon as you give up and let the key off, it starts right up. Every time! It also backfires really loud when you rev it up high (won't always rev up, though, and when it does, it still acts like somethings plugged up). Don't know what else to do. Thanks in advance for your help.