couldn't tell ya, i've never dealt with them. i have put a prochamber from a 01 stang on a 98 though with no problems. i don't see why there would be a difference with the h & x's. the manifold connections are the same & the tranny's are nearly the same size. so if you've had a problem i dont' know why you had it
Being positive before you buy is a good thing to practice. But look at it this way, if you're getting a killer deal and it doesn't work then you should be able to sell it and get your money back if not more.
If you spend over $50 (i think excluding any shipping charges, but i may be wrong) you will get FREE SHIPPING. So if these extensions dont cost enough, maybe you want to buy something else while you're at it, and get some free shipping.