348-409 exhaust manifolds question

The 348-to-409 swap is progressing slowly, but it is progressing.

Once I got the 348 out of the truck, and the 409 onto the mounts, I was happy to see that that the exhaust pipes appear to line up to the manifolds.

BUT the flanges are different. The 409 manifolds are 2.5 inch and the 348 manifolds are 2 inch. The studs on the 2.5" manifold are farther apart than on the 2" manifolds by a tiny amount and I could possibly file the holes on the 2" flange a bit to get the studs to fit, but that would be a chore. And I would have to figure out something for a gasket to span the difference.

Question 1- Could I put the 348 manifolds onto the 409 heads temporarily, to drive the truck to the muffler shop for new flanges? (I'm thinking that I could take the new manifolds along and have them installed there.)

Question 2 - Once I do get to the muffler shop, how important is it that I replace the nearly new 2" exhaust pipes with 2 1/2"?

The 348 is a '58-61 250hp.
The 409 is a '63 QA

Fathead Racing

Well Known Member
Supporting Member 7
If it’s performance your looking for I would replace the whole system with 2.5”. If you are pleased with the performance once you have the flanges in place with the 2” system leave it be. If the engine is a mild build with low performance heads putting on a 2.5” system probably won’t get you enough to feel it in the seat of your pants. Just Remember, this is a disease with no cure if it’s performance your after.


Well Known Member
I would think the runners are also a little larger coming down to the 2.5" flange. If you weren't looking for more performance then why bother changing a 348 to a 409:dunno, do the exhaust right.
Well since it's very hard to find a good RH 409 or 348 Hi-perf manifold at the time unless I wanted to pay like a million bucks for a good one. My Dad tossed out my lo-po ones years ago (thanks dad) so this was the alternative. Besides I am not a pocket book car builder, if I need something and it's out of my price range I will make it if I can and that's what I did with the manifold. Now that I have secured the RH one for the car along with a nice set of Doug's headers, I will take the drivers side port it and sell the pair for more than the $35 I paid for them.


Well Known Member
.I reread your post #11 and saw you answered my question there.As far as the individual runner size From the looks from the outside I can't see where there could be too terribly much difference,at least as far as comparing them to the short 348 HP and truck manifolds. The major bottleneck would be at the outlet.
Last edited:


Well Known Member
Back to the original post,if the muffler shop that you plan to take It to is the same shop that put the 2" exhaust on it,if he didn't try to talk you into putting a 2.5" or at least a 2.25" system on it then,I would be looking for another muffler shop.There was no downside to running up to a 2.5" system on even a stock 348" engine aside from the small additional expense,and would have the capacity to support exactly what you are doing now.If that isn't part of his philosophy then you don't want him designing an exhaust system for your rig.


Well Known Member
Will this work on LH manifolds too?I have considered doing this but wasn't sure if it was possible,did you use anything for a template?Also how far in did you go?
Sorry this is late, yes it should work on the LH side as well. Like I said it was easy peasey. I wish I would have had my new AHP welder as I would have welded extra metal on the outside. That welder with muggy weld will weld cast iron down to 50 amps.