[demian5]

The 400 will outlive the truck behind a mild 350...
What rear end ratio do you have? I'm thinking a 3:73 or 3:42.

But keep in mind, if later you are driving down the road at 3200 RPM doing 75 MPH - wishing you had installed a 700r4, and hear that re-gearing the rear end to a 3:07 will help get the RPM down at highway speeds (which it does), that is $1000 alone.

Factor that into your decision.

I personally have a 350/350 with 3:07 rear gears and cruise at 2500 RPM at 80 with 31" rear tires. Im happy.
I lost the pinion gear on my 3:73 and had to rebuild the rear end anyways, and the 350 works great, so that dictated my decisions. I scored a low mile ring/pinion and carrier - which never happens.