Ok, this isn't talking from truck experience but rather personal experience with my '96 Z-28. If you get other manifolds (why wouldn't you get headers instead?? They'll give more power) have a Y-pipe made. I haven't seen any available for Dodge vehicles but I know they make them for Camaro's... might be worth checking out with Flowmaster/BBK/Borla/etc... (any exhaust company that makes a cat-back exhaust might make a Y-pipe for your truck). The stock Y-pipe typically is very restrictive.
I think what my dad planned on doing is cutting the Y-pipe before the 2 pipes meet to make it dual exaust. We don't know if headers will fit on my truck bc i did an engine swap. I put a 318,727,NP208,Dana 44 front, and 9 1/4 rear from a '82 Ram Charger into my '90 Dakota. The frame on my Dakota was 2.5 inches narrower than the RC's. We know the Y- pipe will fit. Don't know if headers will. So I don't think i'm going to get headers. But I would like new manifolds to get more power and to go with the dressed up engine.
Here's an idea that would work for you. Since you don't have inside frame clearance why not make/buy headers that will run on the outside of the frame? I'm gonna run headers like that when I get to Arizona. They look real trick too because you can polish them up and they are easier to keep clean/shiny.
All you have to do is cut the flanges off your old Y-pipe, and take them to a exhaust shop and have them make (or do it yourself) the dual exhaust setup from there. You could weld or get clamps, but if you go with clamps I have to recommend getting "band clamps" they are bands of steel/aluminum? that wrap around the seam instead of your normal clamp that just pinches everything.
You shouldn't really have to worry about bolts breaking. I pulled the factory manifolds off my '79 Dodge (willing to bet that its the first time they've ever been off) with no problem at all. Didn't even have to use WD-40 or penetrating lube.
I was not sure if the bolts would break or not bc my friend took the manifolds off of his '89 Ranger and he snapped a bolt off. Maybe that's bc it's a Ranger. If they didn't break off of your '79 318, then I wouldn't think they would break off of mine. When I get new manifolds I will find out if they bolts break or not.
A "shorty" header is a header designed to fit onto the stock y pipe for any particular vehicle, as opposed to a full length header which would have to be welded into your existing exhaust setup. "Shorty" headers are made by nearly all the big name companies ie. Borla, Bassani, Gibson, Heddman, etc. I know that for sure, Borla offers lifetime warranties on many of their headers, and i believe that Gibson does the same. I would just suggest that whatever you choose, go with something from one of the big exhaust companies and use high grade fasteners like ARP. That way you shouldn't have to worry about any bolts breaking.
As for taking off your old exhaust manifolds . . . soak the heck out of the bolts before trying to loosen them . . . so you don't strip or break the heads off. Anyhow, good luck.
Someone has recently done a flow comparison between differnt mopar manifolds and headers. Aparently the older Magnum manifolds flow quite well, but the newer ones are a bit more restrictive, but they still flow well compared to headers.
The "shorty" headers will move the torque peak up into the RPM range, well above where it is now, and probably you'll lose some bottom-end.
Running the stock manifolds with a 3" mandrel-bent y-pipe will get better low-end torque than factory. Also running the full-length or Tri-Y headers will get you more low-end torque.
Running dual-exhaust will only help in the uper RPM ranges. The best for torque is to go a 3" y-pipe or full-length headers with 3" collectors and y-pipes, and then run a single 3" all the way back (with cat and muffler of choice).
If you're going for HP, then run shorties and a full dual setup (providing you don't need to worry about emissions stuff). For torque, full-length headers if you can get them to fit (not likely), stock manifolds otherwise, and 3" out through the muffler.
i learned that from experience. when i put my first charger motor together, it was pissing out the header bolts because they didnt seal the thread good enough. a little radiator stop leak fixed that though.
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