So, late coming to the thread but have been drag racing ...forever... I'll throw a different perspective on this.
Instead of looking at what you got and trying to get where you want to be, see where you want to be and build to get there.
You want a cheap 13 sec 4wd truck.
Squeezing a little 4cyl might (could?) get you there, but for how long and what would the drivability be? (1200rpm idle, peak hp at 7000?)
351/4bbl will easily make the tq to do the job...but, will you just find the next weakest link? (blew the t case, replaced it, blew the tranny, replaced it, blew the u joints, replaced it...etc...)
I would buy a used 4x4 150/bronco/etc with what you want and transfer everything over. A lighter ranger will be easier on parts than the donor vehicle and if you need to bump the hp a bit you still won't get into parts breakage territory if you're only running 13 seconds.
Also, do you want it to be consistent 13 sec or a "pull the trigger and look at the score board to see what you run" kind of truck? Small eng/turbo/nitrous/etc will all lead to inconsistent runs. Yes it might turn the number, but a 13.2, 13.9, 13.5, is only good for test and tunes, not racing. (unless that's all you want)
Another consideration is hp required. Yes the number you quoted in theory will produce those time slips, the problem is, ET is based on suspension and trap speed is based on hp. So, X weight and Y hp is never quite right when you get to the track because you don't have optimum gearing/traction/weight transfer/etc. I've seen a 450hp (on the dyno) run in the 13's and I've seen a stock olds 455 with probably 200hp in an old flopper body run low 11's. Calculators are pretty close in a race set up, not so much in a stock set up.
$250 you can get a 3.75 stroke crank off ebay that uses 302 flat top pistons to make 396 cu in. A set of GT40P heads and .512 lift/230 dur @ .050 cam with 373 gears and low, reliable 13's will be no problem. Plus, the torque will blast you out of holes from idle on up to 5500 and make you think you're running a big block.
Anyway, my $.02