going through your options as you listed them:
- Another F-series motor.
Since you have no smog bullcrap to square off in a 70, I would advise you seek out a 2F that's in good shape, or has been rebuilt, desmog it, and drop in a 4speed.
-If you're feeling really crazy, you can get a late 2F block, 3FE head, and make a 2FE, and never worry about off camber (which is unlikely as you said you don't have much offraod interest.) You'll also see improved fuel economy over carbed, and it'll be a little less of a maintenance hog. (Also a 2F running a TBI setup will work, see Trollhole's "Big Gay 2f Build" Thread.)
-Another alternative is to dig up a Toyota 13BT turbodisel motor and H55F 5-speed, and have some of the best fuel mileage you could ask for in a 40, but said motors can be a little bank.
Those are all the practical options that don't require you to lose your "Toyota" emblem off your grill.
-SBC options.
These things are cheaper than any F-series motor mod, and here in the States, we have Chev parts coming out our ears. Adapters run about 500 bucks to mate an SBC to a 3-speed j30 transmission, and they work, but the older trannie's don't care for the added high-end stress, and you have all the limitations of a fairly primitive 3-speed.
If you feel like going crazy, you can also swap in the chev transmission, and overcome these issues. SM420's and SM465's bolt right up to a stock Chevy BH, and fit within the limited space of the 40 wheelbase. Adapters are to be found. Automatic transmissions (TH400, TH350 4L60E etc.) also work, but they're a colossal PITA to get set-up right, and they're kinda like buying a used snorkel, you'll never really know just what the PO ran through the holes. (and sometimes, you'll never really want to know)
You can also do a Ford swap, but things aren't as cheap with Ford tech because GM's slogan is "Quantity is Job One".
-22R(or any such R series motor)
However awesome these little motors are in a mini, they're terrible in a 40. for starters, there's not a one means of adapting one of these to an LC transmission, so you'd have to swap over axles, driveline, motor, and spend tons of coin getting everything to work right. Additionally, a 40 weighs fair amount more than a mini (including stock driveline, axles, body and frame), so youll have to really work the motor to get your rig anywhere, which means, there goes any hope of fuel economy.
Best of luck.
