Originally Posted by Festus Hagen

The 87 6.9 is the preferred motor for performance building, though late 85, 86 motors are just as good with the addition of 7.3 rockers.

Both the 6.9 and 7.3 need head studs in order to raise the boost levels, there currently is no TRUE stud kits for the 7.3, so ultimately you can boost the 6.9 higher.

Installing a stock 7.3 for performance will gain you little if any.

I believe Hypermax is still supporting the IDI.

-Enjoy
fh : )_~