10's on $5,000 ?

I also like the short box Dakota plan.

My only concern would be headers.
Are they available cheap?
Are cheap headers close enough to fitting that making them work isn’t a huge hassle?

A “nice” 65-77 B body will be easiest in the long run........ unless you stumble across a 4dr Dart or something.

I’m gonna throw this suggestion out there now.......
Look for a deal on a used tunnel ram.
Most of the guys running big blocks in dakotas are using hooker fender well A headers. Being a full frame deal, one of A&Rs motor plates and a solid transmission mount may give enough clearance for a set of chassis pro type headers, but it may take some frinding on the block and frame rails to pull it off. Someone recently introduced me to a strange local phenomenon in our nearby Facebook marketplaces. We are ground zero in a 150 mile radius of a scourge of two wheel drive Dakotas of late eighties to mid nineties vintage. Most aren’t running and can be had for $350-$500.