[SPJA]

whats the widest pipe i can put on my 1975 Mustang II Mach I exhaust? like a 2", 2 1/2", 3"? that would give me BEST PERFORMANCE

[heavy metal]

2.5 is a good size to run,plenty of room to breath.and i suggest an x or h.

[SPJA]

is it possible to fit a 3'' pipe in there?

[STANG II]

3" will fit but unless you are heavily modded you won't benefit and will loose bottom end torque, 3" is mighty big for a 302.

[AT68]

Losing torque from an exhaust system that's too big is a myth. The only way you would lose anything at all is if the free flowing exhaust caused the motor to run lean, since opening the exhaust up has a slight leaning effect that might need to be corrected.
Ideally, zero restriction in the exhaust is what you want, and yes, 3" pipes fit, because I've had them on my car before, with 3" 2 chamber flowmasters. (I run 2.5" now and 3 chamber mufflers to try to control noise)
The only place where pipe diameter can be too large(or wrong length) is in the header design itself. Proper primary tube length and diameter, and collector size are critical to engine performance, but the exhaust after the collector should be as big as will physically fit on the car to maximize performance. (unless noise is an issue, or you're looking for a specific tone)

I'm actually installing 3.5" duals with the new motor.