High rpm for mph

[happyhunky]

Hello, I'm just finishing up a project started by someone else, and has been sitting in storage for 6 years. It's a 57 chev pu on a shortend 76 suburban frame. It has a rebuilt 350, rebuilt th350 with shiftkit and stock torque converter, backed by a np205 transfer case, rebuilt 14 bolt rear with locker and rebuilt dana 60 front with posi running 4.56 gears. The problem is that now I've got it to where I can run it on the road I've found that it is turning 3000 rpm and going just 41 or 42 mph. Please tell me if I'm not right on this but with tires (285/75/16), a np205 in hi range running 1.00 ratio and a th350 in 3rd gear running 1.00 ratio I should be going roughly 60mph. No overdrive and no underdrive should be running straight 4.56 final right? I am sure it's not in lo range cause I feel the difference between lo and hi. And I know it's shifting into 3rd gear both manually and automatically. My timing is 8btdc and my plugs are showing perfect for fuel mixture. Starts and runs great other than high rpms for mph, and still runs warm some, but think that's still just from being rebuilt. Could you give some ideas of what to check or could possibly be the problem. Everything seems to work like it should, kickdown, shifting, fluid level, etc. All input would be greatly appreciated. Thanks