what size sending unit?

Stock size would be a 5/16" unit. But a 3/8" would be an upgrade...especially if you go up in motor at some point. You can get 1/2" too...but it's more expensive and not needed unless your really pushing 600hp +...

You can get them online. I got mine from Quanta products...but Vans Auto has good prices as well as maybe even Rock Auto for a cheapy replacement.

Re. the filler neck, I'd assume you just need to siphon down the tank until you can remove the neck w/out spilling gas....you'll want as little gas in there anyway to drop it and put a new sender in anyway.