Sounds like you have enough modifications planned that the body is the important part and any model could be made into what you want, won't be much of any original drivetrain left.
I'm not against modifying things... But it is a good plan to drive one as it is for a while and then make those plans. Quite possible you won't need or want all those modifications, which may save time, money, and be the difference between having a running driving truck and one that never gets finished or on the road.
Totally agree, finding a solid running 3100 locally $3000+, a running and driving 1 ton + farm truck $1000+ locally. The farm trucks are generally in much better shape too...
If I'm gonna resto-mod an AD truck it makes sense to just start with a running 1 ton. and...
