If you have a stock type sender, you need a cork/rubber gasket. You can get a GM gasket from Greg Coleman, who has a perpertual ad in the regular classifieds (not the supporting member classifieds) at chevytalk.com.
There are other sources. Roger55 posted a pic of what looks like the right gasket a couple of days ago.
You need to use a non-hardening sealer on the gasket. The old standby Permatex #2 will work, but there's better stuff out there.
You either need to use sealant (Permatex #2 works) on the screw threads on the sender screws, and/or use the Danchuk (or locally sourced) copper sealing washers under the screw heads.
The gasket does not have an evenly spaced bolt hole pattern. It is irregularly spaced. There is a notch in the gasket and it goes at 6 o'clock, there is a dimple on the sending unit that matches the notch. If you don't have the gasket oriented properly, it will pooch and leak.
Don't overtorque the screws, even the good gasket will pooch. A thin plain rubber gasket or a thin cork gasket will pooch at normal torque. That's why you need a good gasket.
Last thing, on your EFI deal, don't assume it's the sending unit unless it's clear that it is. Jemo posted a few days ago with a similar complaint and he had problems with the hoses and fittings, and fixed it by using the right stuff.
This is an area where attention to detail and knowing what to do pays off.