Putting a 207 in a car that originally had a 255/7 in is going to be a headache and will require an aftermarket
ECU.
The 2.5 heads dont physically fit as they are too big for the 92mm bores on the 207 shortblock. So then you're looking at swapping heads over which brings its own problems. The 2.5 heads use different sensors and the cams have different trigger patterns.
That's not to say it cant be done but it's complicated.
Also judging by what you've written you need to do a bit more research. Your combinations are all over the place. The swapping short blocks and thicker gaskets thing is for people putting in a 2.5 short motor using 2.0L heads. You're wanting to put 2.5 heads onto a 2.0L short block which cant be done because the physical size of the chamber on the heads is bigger than the 92mm bore of the block.
If I were you i'd either rebuild what you have or just buy a EJ257 short motor.