imo, I wouldn't even re-ring that magnum if it ran great at 140k and had good oil pressure. I'd slap a intake/carb on it, and I would put the LA front on it with new chain/gears, and if running a mechanical fuel pump, put on the eccentric adapter from hughes or swap the cam. Going to a car oil pan (with a new rear main seal) is a given. Punch in some new freeze plugs and enjoy.
I have a 318 mag with 134k that I will do exactly what I listed above and I'm going to install it in one of my dusters. I will put a 904 behind it.
Shoot, my truck with a 5.9 mag has over 228k on it and I'm not afraid to burn the tires or turn the rpm's. Runs like a top. Guy at work has 264k on his and beats on it, still runs great. Many, many out there with 250K + and some well over 300K still purring like kittens.
* Also, Dodge 72, if you run a 904, you won't have to shorten your drive shaft again. Only if you go to the 727.*
