Help! What am I missing?

[Madbomber88]

I bought a rough, bone stock (as far as I can tell) 72 K20 with a 4-speed manual. It came with the Eaton HO-52 and 4:10s with a locker. When I got it (had it shipped, so wasn't able to check it out beforehand), it was revving around 3600 while driving 40 MPH...not good

Since gears for the eaton are basically non existent, I tracked down a 14 bolt with 3.42s and an eaton limited slip. Put that in this weekend, and now it revs about 3000 at 40 MPH. Am I missing something, or does this not make sense. The clutch is not slipping, and the tach is accurate. What else could it be, or is this normal? Was there any gear differences in the SM465s or NP205s? The number on the NP205 is C-99404.

I understand this is a truck, but I need to at least be able to drive it on the highway. What does everyone think? BTW if anyone wants the eaton rear, it is available and in very good shape.

here's a pic of the truck