Why is the 6.2l v8 swap in my 1965 mustang glitching out? Need for speed heat.

I tried that, it seems to only be the engine itself thats glitched because I can swap the parts on it but can't change it back to the original engine because when I do it changes back to the 6.2 on its own and still acts like it's got the stock engine even though it has elite parts on it making 800+ horsepower and struggles to get over 100 mph.