Drivetrain 1974 v6 to v8 swap questions

What size driveshaft are you running with your T5 conversion in your King?

The 4 speeds supplied behind V8 and V6 Mustang II's were barely strong enough for the 135 hp the 302's put out. (mainly the torque that wrecks them anyway, and the cases flex under any serious load.

Most guys will tell you they have gone through a few if they have owned their car for very long..

The V6 trans has way too low of a gear to work properly with the V8 unless you have like a 2.00 rear end gear. First is like 4.0 to 1. all that torque multiplication and a V8 equals transmission confetti.

I am pretty sure the V6 is the reasonable power limit for these transmissions in reality. The V8 cars, if driven lightly can survive, but there will be no clutch dropping or crazy shifting or you will be looking for a new one, and they are not easy to find.

The simplest things and cheapest in this order are leave it a V6, or do the V8 but with automatic, or do a T5 swap as someone mentioned earlier, although I would guess you don't want to drop the coin required for a quicktime bell housing..

Stock V8 4 speed stuff for these cars is not cheap or easy to find.

IF you had a good v8 4 speed drivetrain donor car, then you could stay stock with that too, but good luck finding one of those cheap anymore either.
Everyone in Mustang land (except us Mustang II lovers) calls these cars down, but that is not reflected in the prices of parts. Good V8 stuff and Cobra pieces cost an arm and a leg.
It's much much cheaper and easier to build a budget fox body.

If you are serious about the swap without driveshaft work, the prior suggestion of the T5 is the way to go. I think the T5 would be good for around 300 ft lbs torque reliably if it is in good shape, which is probably double that of the RAD 4 speed.
Keep in mind you also only have a V6 diff and V6 driveshaft, and I believe both are lighter duty than the V8 cars. SOME of the V6 cars had the 8 inch diff, but a lot did not.