So to recap, I'm building a fun 71 Demon. Dropping in a nice warm magnum 318, with a 4bbl and nice '340' cam, and an A-833od 4-speed, and 3.23 gears - just a nice clean-running hot street machine that cruises perfectly on the highway and idles with a bit of lope - probably a 5500 rpm limit. Looking at the Whiplash cams from Hughes. Could care less about the engine making 'maximum power' - all I care about is the engine making nice power without sacrificing streetability and don't want it to be overly smelly - no 108* cams, and no headers - want things OE-looking and OE-sealing. That said, here's some additional thoughts on suitable exhaust manifolds.

To me I basically have three options: 1) '02 Magnum manifolds, 2) 340 manifolds, and 3) 318 manifolds.

The magnum manifolds have significant physical interference issues with factory A-body power steering, steering columns (all), and z-bar linkage. I am probably going to run a hydraulic clutch setup which I think will cure the z-bar problem, but unless I can find a nice aftermarket K-frame setup with power rack and pinion steering I am not seriously considering magnum manifolds.

340 Manifolds would flow more than enough...but I don't really think I will 'need' 340 manifolds....and for darn sure I don't want to pay the huge $$$ for them. Maybe.

So...why not 318 manifolds. Check out the port sizes in the below pic. The 318 manifold ports match the magnum head exhaust ports perfectly - almost exact. That alone indicates to me that 318 manifolds 'match' magnum 318 head ports, and that has to count for something. 340 manifold ports are HUGE compared to magnum ports...which IMO also probably means the huge ports and internals are likely 'unnecessary'. I already have a set of 318 manifolds...and so I am seriously considering running them.

The car I'm cloning had a warm 318 with 360 heads, and I think I had 340 manifolds on it - I honestly don't remember, but they were either 318 or 340 manifolds, but definitely wasn't headers. The thing ran great. Assuming the same cam profiles, I'm inclined to think an LA 318 w/ 360 heads and 340 manifolds would run nearly identically as a similar magnum 318 with LA 318 manifolds, or at least so closely the same that I'd never notice the difference...at least not at/under 5000 rpm.

Meanwhile, I'm pulling the 318 manifolds off today and will measure the outlet port sizes, and I'll measure the 318 exhaust port sizes for comparison to the magnum ports. Hopefully the outlets are at least the same as the 1-7/8" magnum outlets. So, I offer these idea to the congregation for y'alls thoughts.