The Magnum exhaust manifolds come in two different designs.
The design for pickups, vans, Dakotas and Durango’s – the rear exit and firewall will want to occupy the same space and are not recommended.
Left
Right
Note: all pictures used on this post, were swiped from eBay.
The Jeep Grand Cherokee, the exhaust exit is between the two rear-most cylinders – and will fit our cars.
Left
I could not find a picture of Right side – but it is pretty much a flip side of Left manifold. There are venders selling the Right side on eBay – but have no pictures are up.
Depending on what year your FMJ was born on, this picture may vary slightly - but the overall shape and exit is the same for Left side.
The right side is not critical and most all small block exhaust manifolds will fit - except for the pickup/van/Durango/Dakota magnums (top two pictures).
I can't be of any help with headers - for I will not use them.
There are several magazines who has flow tested Mopar small block exhaust manifolds, and the best flowing is the ’68-71 340 – but those manifolds are not easy to find, nor cheap when you do find some. They are being re-popped – if anyone is interested and also have money to burn.
This 340 Right side manifold might be one of the best ever exhaust manifolds ever made, possibly.
The Left side goes up and over the steering gear.
It fits FMJ cars and I have seen it on a few of our members cars – but looks out of place (in my opinion).
I have a pair of the above manifolds in my garage and a strong likelihood of using the Right side, one day.
The Left side - I have no plans on using and it is for sale (fairly cheap) if anyone is interested (casting number 2863553).
The second best flowing small block are the above mentioned Magnums.
The early (Magnum) years – the exhaust exit has a larger diameter hole. No mention in flow tests if this works better or not – but if I was going to look for Magnum exhaust manifolds – I would look for the early years. I left my laptop at home today – so don’t have my casting number data on hand to mention here.
In May 2012, I purchased an Left '94 Grand Cherokee 318 manifold, casting number 53009477 for the incredible amount of $13.75 (which included freight) on eBay. This manifold has the 2-1/8" diameter exit.
One of these days I'll install that as well as my Right 340 manifold onto my car . . .
BudW