With the GT-40X cylinder head's larger-than-stock exhaust ports, the stock crimped-tube manifolds are choked. This restriction makes using a proper 15/8-inch, mandrel-bent, short-tube header a must. FRPP has just the part in these M-9430-P50 stainless steel short-tubes, which fit great and are built to last. As a bonus, there's never an easier header installation than when the engine is out of the car. As a caution, we didn't like the fit of the supplied gaskets (bolt hole misalignment) and stock gaskets have under-sized port openings, so we used Fel-Pro 1415 gaskets, which fit in all dimensions, were thicker, and obviously of high Fel-Pro quality.