The performer was designed as an emissions era stock (1500-5500rpm) replacement 4 barrel for the 318/360. It's not really a true "high rise" manifold like the OEM 340 manifold (which by the way still works very well for what it is). If unmodded it should have 318 sized port entries that can be opened to match the 340-360 sized ports as well.

But for what it is, it's a decent piece (i.e, it doesn't suck) and the manifold you can get for $70.00 sounds like a slam dunk. But for a real High performance motor the RPM or the LD-340 would be the way I'd go. You have to watch some of those old edelbrock castings, the aluminum can be very brittle and easy to strip....very inconsistent metallurgy in the 70's IMO...you either got very good or very bad aluminum.

To build a high MPG 318 I'd use a set of the KB 9:1 flat tops with a zeroed deck set for .040 quench with your gasket and find a set of the late 80's small Heart shaped chamber '302 castings with a little bit of seat and bowl tweaking and an SP2P manifold and with some radius work in the plenums and use an oem 360 .410 lift cam. built right with a lock up converter and 2.94-3.23 gears you could probably squeeze 28mpg out of it in an A body and it would go for 200,000 miles.