Okay, let me try to address this one thing at a time. I'm going to assume you have all the tools and money's not a huge concern. Your 75's problem is gonna be behind the pump, yes. I would drop, pull, and check everything before you left. You could very well be right on the lines being cracked and sucking in air, but I wouldn't rule out degradation of the little rubber lines and clamps on the sending unit, especially if the truck sat for any period of time. Also, the strainer sock could be inundated with filth. It could be a contribution of all of the above, too. I'd drop the tank or tanks and get a better idea. You'll see the rubber lines, you can pull the sender, and you can clean the tank. I've done this one of mine, and I'll be doing it on my other one soon, and it's worth while. You didn't specify which setup you had on the tanks. Same goes for the later model truck, but I feel like that might be more a strainer sock/sender problem while the older one is cracked up lines. Honestly, I'd figure this stuff out beforehand so you don't have to make more than a trip. Depending on your tank setups, get you an appropriate amount of 3/8" fuel line from the store. It'd be good to get extra in case you mess up or something. If you need a new sending unit, do yourself a favor and don't buy it from the store. Their prices are highway robbery compared to RockAuto or even eBay.
I don't understand the whole deal with the carburetor. 49 State trucks were not computer controlled this year. California trucks had Computer Command Controlled E4ME Quadrajets, plus the whole basket of OBD1 goodies, but that was the only state. Those are identifiable by an oval shaped plugin for a TPS sensor on the body of the carb and a plugin for a Mixture Control Solenoid on the air horn. Some of the mechanical ones, though, could still have that exact same plugin on the air horn for a dual stage accelerator pump. Anyways, everything else was an M4ME (mechanically controlled, electric choke) or an M4MC (mechanically controlled, divorced choke) Q-Jet. If you could take pictures of the old one and the new one for reference, that'd be good, but unless it was originally a CA truck, that was 100% unnecessary. If the TV cable geometry is true, your 700R4 will be fine, which it should be. Those problems only arise when swapping for an aftermarket carb.
On the distributor, it sounds like something's wrong with it. You can certainly bypass the ESC per the procedure and run this distributor, and I would do that if you don't want to make the ESC work, and that distributor treated you well before it started messing up. I don't think it could just be the ICM. Your pickup coil in there could also be the culprit. Are you saying it just detonates when it's under acceleration? Can you get a good base setting on it at idle? It sounds like this distributor is in better shape. The pickup cool and ICM clearly work, your cap and rotor are clearly conducting spark, and your vacuum canister and plate sound like they're doing their job. It may just need a set of recurve springs put in it or something. You've got options here. Honestly, if it were me, and I tend to do things most people on here wouldn't waste their time with, I'd try to get the ESC working on the original distributor. 305's can be little ping-monsters sometimes, and if working properly, that serves as something to protect your motor. It's up to you, though.
