If you look the ad over closely, you'll detect an oxymoron. "Dramatically improves low-end torque, fuel economy and engine performance from 2,500 to 7,000 RPM." You can't possibly do that since low-end is 2500 rpm and below. For the record, unless one is racing or just has really low gearing, 90 plus percent of the time is spent at 2500 rpm and below. You can't gain anything if you don't operate in the intended range it was designed for. In fact, you'll stand to lose in the everyday 2500 rpm and below range. I tried to cross this manifold in my Offy catalog, but JC doesn't use Offy's #'s. Since they only make two different intakes for the AMC I-6, I suspect it's their "C" series manifold. Their other manifold, the "Dual Port" is designed as a dual plane intake for improvements in the 1000-5000 rpm range, right where the majority of us operate. Before I replaced my Buick 225 V-6 with a Buick 252/4.1L V-6, I was running the Offy Dual Port intake, Holly 390 cfm 4 bbl and HEI distributor. It was a very nice combination once it was properly set up. I suspect the same combination on the AMC I-6 would prove likewise. Don't waste your time with headers as they too don't benefit until above 3500 rpm. Stock manifold with good single exhaust, (Flowmaster or the like), exiting behind rear wheel should give the best benefit unless you intend to race most of the time. Good luck.
'66 Jeepster Commando w/Buick 252/4.1L V-6
'70 Wagoneer w/Buick 350 V-8