The only downside to an HEI is if you're doing any kind of high RPM stuff... they start dropping serious voltage after 4,500 RPM's. For most cats this ain't a problem since their engines never see that many R's at all. Some never.
But for me, where my engine will see 6,000+ RPM's quite a bit... some kind of Aftermarket HEI was in order. I use an MSD Pro Billet unit. Anyways... Just a thought in case anybody is puttin' an HEI in a racing, or street/strip rig.
Gary