340 stock manifold to header adapter question

Steve, if you have time to wait on your down pipes that fit the manifolds, maybe an exhaust shop could make you a flange to mate the 2 temporarily. If you have the flexibility to move the pipes a little bit on the run stand, you may be able to just buy a 2 1/4 to 2 1/2 inch pipe adapter at the parts house to mate the 2 pipes long enough to run the motor in. When we ran my 273 on the dyno, an exhaust shop close by made me an adapter so we could hook the dyno to my headers that I took to use. He made it out of 2 pieces of 3" pipe and two old 3" flanges that had the 3 bolt pattern. Worked really well.