I have the option to buy a 1967 mustang 390GT cam and timing chain with gear.It is new in box for a good pricce. My question is would it improve the performance on my stock 390?

 

Yes, although you may want to clarify what you mean by 'improve'. It will move the peak torque higher in the RPM range, and that will make more peak horsepower.

I remember my friend's 67 Fairlane 390 GT in high school. Gawd that was a fast car. Although it was factory rated at 330 hp, it was considered underrated.

390 GT heads were better heads than the standard 390 heads of the time. I think that the D2 truck heads can be made to work like the 390 GT heads. I bring this up because a hot cam works better with good heads.