It's odd your sending unit doesn't work from 521 restorations because both of mine are from there and mine still work great after 5 years so far and I noticed they are made in America so its odd yours failed. Could it also be why your truck isn't running properly? I've seen sending units cause stumbling due to sucking in air.
the first time I put fuel in it...It read Full when the tank was full, but by the time I had used 10 gallons, it was down to a 1/2 tank so I figured I got a shit unit. Now that it's DOA, I'm thinking that the float failed. I've already got a new stewart warner lifecell float as I bought two of them when I fixed my 70 R/T.
I emailed 521 Restorations about it a while back, but never got a reply, so more than likely I'll never do business with them again. I should have NEVER thrown away the original sending unit and I should have had it restored instead....it was rusty and ugly, but there's a few people out there that will either restore it, or restore and transfer over the float part to a new stainless sending unit.
Either way, even though it sucks that I have to drop the tank, I'll finally get to add a dedicated return line like I should have done the first time.