Hey Fred,
I’d only put in a 302 Windsor, you’ll have much better room and they rev nicely.
As Dingo and Geoff say they fit but only just!
In a 66 Mustang with a 302 it will have all the power you need, unless you want ridiculous power.
In my experience don’t fit the later EFI 5L engine, unless you want to change it back to a carburettor, and remove the ECU.
If you keep the EFI ECU you can not add many modifications, the ECU won’t adjust to suit!
Saying that you will need to fit a new intake manifold, distributor and carburettor, You may also want to fit Alloy heads, new headers, Cam, bigger twin exhaust, add Borg Warner T5 gearbox and 9” housing and LSD. Plus you will need an Engineers certification, the price is adding up!
It all depends on what you want, but it could cost you heaps.
I stroked my 5L EFI to 347ci, then you worry about breaking the two bolt mains, in hindsight I would have felt more comfortable with a 331ci.
I have Unisteer power steering and had trouble getting the right headers to suit it and the transmission Z bar.
And I have enough trouble changing the spark plugs.
But I don’t want to turn you off what you want, it is very rewarding!
Whatever, enjoy your car.
Cheers Phil.