My aftermarket World heads have 3/8 NPT holes for a sending unit. The POs sender was the wrong range. I ended up using a 1/2 NPT sender and installed it in the intake manifold. NAPA Echlin has a catalog you can wade through (it is a beeeyeteeseeaych to download
http://www.napaechlin.com/upload/NAP..._NA141_HQr.pdf) and perhaps find a 3/8 NPT unit that will work. I gave up and went to 1/2 NPT.
Check Posts #19 and #20 in my project thread.
This isn't the solution if you want to stick with a 3/8 NPT unit, but I hope it helps you a little.