As a word of caution, I put an ipd header on my '70 144 and i had nothing but trouble. first the manifold surface was machined improperly. it was not flat and did not seal to the manifold. second the header would hit the lower A-arm at the back bushing. this may not be a problem with 1800 suspension, i really don't know. all in all i was dissatisfied. but maybe i just got a bad header. i know grassroots motor sports (grassrootsmotorsports) had one on their 122 and had some trouble. i cut the oe exhaust pipe, added the reducer as ipd indicated and retained the oem muffler.
i do like the weber dgv carb though, it's a big improvement over the wornout strombergs.
I hope everything goes well, Brad M.