why do you want to go with chevy leafs if this is a street truck?
what 302 is this anyways? there are 3 basic 302 efi's your mustang 302 efi (ho) your explorer and your f-series 302, they are all different, and all have advantages and disadvantages...
between the explorer and f-series, the difference as far as i know, is the intake, but i would assume the cam is different also, giving more low end torque to the f-series, for towing.
the mustang (ho) motor uses a different firing order, it does NOT use the 302 firing order, it uses the 351w order, which eleviates stress on the bottom end, and allows more power to be put through it, with less wear...
anyways, your motor can be converted to carb very easily, by mainly just changing the intake manifold, and adding a carb, personally, when i build the motor for what ever truck i put it into, i will be pulling the motor out of my 89 mustang, and putting an edelbrock rpm airgap manifold on it, with a holly carb.
you might want to look into a new cam, or different ratio rockers, to give you a little more power...
tranny, without a doubt, t5 transmission! if and when the transmission in my 07 goes on me, i'll be doing what i have to, to get a t5 put into it, the t5, in my opinion was the best transmission that ford ever made, they are bulletproof, and hold up against the abuse of thousands of 18 year old kids driving their first muscle car
as for the 8.8, the 28 spline (what most rangers have) will be fine, up to a certain point of power, but honestly, i woudln't worry about it too much... the 8.8 is what all of the mustangs between about 78 and 96 have (and possibly mustangs out of that range also) and a lot of them have 300+hp put through the stock t5 and 8.8 (though i don't know the spline count on the mustangs, i would assume it's 31)
does that answer your question?