The R/T pickups use regular pickup exhaust manifolds. Now the exhaust after the manifolds might be different (not checked).
Magnum exhaust manifolds come in two different flavors (pickup, Dakota, Durango & B-van vs. Jeeps). For simplicity, I will use Dodge (pickup, Dakota, Durango & B-Vans – which are all the same) vs. Jeep (Grand Cherokee)
’94-03 Dodge versions:
Left (Cast # 53010187)
Right (Cast # 53010186)
Note: The ’92-93 pickup versions (the last two years of production of the ’71-93 trucks) – look close to the above two parts. I doubt they will fit - but one or both might (I just don't recall what they look like). Casting numbers 53006618 and 53006619.
’93-98 Jeep versions
Left (’93-98 Cast # 53009377) - 2-1/8” inner diameter
Right (’93-95 Cast # 53009376).
Right (’96-98 Cast # is not known).
The Dodge version – the rear of exhaust manifold (either side) outlet will want to occupy the same space as your firewall - so modification of firewall will be needed (so not advisable).
The Jeep version outlet is between the two rear cylinders and fit fine in all older (~’60-’89) RWD Chrysler passenger cars, I’m told, except for some A-body’s.
The early Magnum exhaust manifold outlet size is 2-1/8” inner diameter. The newer Magnum outlet diameter is 1-7/8” diameter – so a minor item to look for. If you have a Jeep manifold with casting number marked above, and it is 1-7/8” – you might be able to spend a little time porting it out a little, if that is something you are into.
Here is a Left side Jeep manifold selling for $24.99 (+ freight) – that I would highly recommend anyone considering this changeover – to jump on
https://www.ebay.com/itm/DRIVER-LEFT-EXHAUST-MANIFOLD-8-318-5-2L-FITS-93-98-GRAND-CHEROKEE-168602/382465582987?fits=Make%3AJeep&hash=item590cb9a78b%3Ag%3A3iIAAOSws-ta%7EdmA%3Asc%3AUSPSPriority%2173127%21US%21-1&_sop=15&_sacat=0&_nkw=JEEP+EXHAUST+318&_from=R40&rt=nc&LH_TitleDesc=0%7C0
(Note: I already have one – just waiting to install it).
One thing to consider is the A.I.R. ports on existing cylinder heads, will need to be plugged before these will work. Another thing to keep in mind is in cold climates, you will need to fabricate a heat stove for air cleaner hot air supply. Those in warm climates (or using fuel injection) will not need to worry about that.
I also talk about more this at this link
https://www.forfmjbodiesonly.com/classicmopar/threads/headers-for-j-body.6361/#post-80339
BudW