Your going to have to try the headers, my car was the opposite do to it having a B1 motor in it before, those headers are 2 1/8 to 2 1/4 step and they fit and worked fine on the M.W. Victors and the new motor with the ported 440-1. I did have to dent them to clear the front frame tubes, it is a chrome moly tube chassis car, do to using a stock 400 block.
If yours won't work I may have another race set that will work in a stock chassis car
PM me if you need to