Although hinted on, no one really listed what would be needed for a V8 Swap.
-1975-1978 Mustang II motor mounts and frame mounts. Motor mounts can be bought new from Ford, frame mounts must be sourced used. (someone here likely has some).
-A II has a II specific C4 Automatic transmission. It has a specific, Bell housing torque, flex plate and starter and seperation plate (tin plate between bell housing and engine).
-Front and rear springs
-Very possibly the rear end. I have only seen one II with a 6 and whatever?? most had the 8" Again likely one of us has a 8" you could buy
-75-78 V8 II rad and hoses.
-V8 throttle cable and down shift rod linkage.
-Mustang II Headers (Headman are the only ones that can be bought new). Many prefer the Dynomax/Blackjack headers that can often be found N.O.S off e-bay). Mustang II exhaust manifolds are pea shooters, best used for balast or paper weights.
-Custom exhaust (this is a whole other topic on it's own).
My sugestion: Carefully remove the original engine and moth ball it in your garage for the future.( A 4cyl Cobra is a very rare car). Buy an XR4TiI or Turbo 4cyl Thunderbird parts car. Swap in the turbo engine. The V8 in a II was an afterthought! It hardly fits in the car and is placed way too far forward and it is not even centered! The Turbo 4 would be an easy swap that could be reversed with ease for preservation sake. The turbo 4 can makes V8HP stock 175hp in XR4TI non=intercooled state and 190hp in intercooled Turbo T-bird 5 spd spec. The car would handle better and have the V8 perfomance you want. The 2.3l Turbo has a big aftermarket following and easy bolt-on mega HP is easy to obtain. I have a V8 car, but many here have swaped in the Turbo 4 so they could give you specifics. I don't think you would have to change anything?? If the car is Auto, it could of had a C3 or a C4. A C3 may not handle the stress of the Turbo. The C4 could use a performance rebuild to help cope. The 8" rear end would have no trouble. The smaller rear axle would not be happy
Mustangs are intended to be enjoyed and I can't see a 4cyl Mustang II ever being a numrically valuable car. Personally I would like to see more II's on the road and have no moral objection to the V8 swap. Guys it could be way worse! Some old geezer could have bought it and lopped off the front
suspension to build a rod and the rest of it would be on it's way to korea to be melted down to make new Hyundai's