A few thing to look at.

It's very common for these cars to have non original and mismatched exhaust manifolds.

I would confirm what manifolds you have by the part/casting numbers or post a few pics.

My 73 E body has a 69 A body manifold on the drivers side.

Another thought.

If the price is right you could just buy a 340/??? set and have a local shop cut the flanges off and replace them if they don't fit your manifolds. Once the pipes turn back towards the rear they are all pretty much the same.

For me the price would have to be pretty cheap to buy a new pipe and slice it up but I have had to do it before.