I think 1500rpm would be good with my 427 and the long flat roads we have here. The Brownie box i am going to use is a Spicer 7231-D
2.14:1-underdrive
1.00:1--direct
0.86:1--overdrive
So i should have plenty of options as far as gearing goes.
5inches of drop at the axle could be as much as 8-10 at the back of the flat bed, depending on how long the flat bed is. I plan on using either the stock air compressor off my 427 TD which was in a C60 if i can get it to fit under the hood. Or i can get a york air compressor. I guess the biggest reason i was looking at the semi axles is because i am cheap.....i know...i should get another hobby. But half the fun for me is figuring out different ways to do something without spending a lot of money. I have less than 4K tied up in my toyota rockcwaler and keep up with the guys in 30k buggys. here is my GF playing around in it.
http://www.texas4x4.org/gallery/Jeromys-Toy/At_the_riverI have an 83 chevy 1-ton 4x4 i concidered using for parts as it has the same D70 HD that a P30 has except i would be stuck with 8 lug 16.5 wheels. I can also get a 97 chevy 1-ton 2wd dually with a vortec 454 and NV4500 for $1200, but again tires and wheels are the limiting factor without spending big bucks. Honestly the possiblities are endless at this point and they make my head hurt.
