I agree-
It depends on the rest of the motor though- with a stock head and cam there is about 3-4HP going from an HS2 carb & manifold to a later HS4 one (as on the automatics). If you cut the inlet off the exhaust, that's worth a couple, according to Vizard and Trickey.
I doubt there would be a heap more with an aftermarket manifold unless the head etc has been done, and like you say the HS4 ones cost zip.
To locate the inlet manifold after cutting exhaust off, I build up the outer end lugs with weld then file to just fit on the studs. You can do this with stick weld (Ni rods for cast iron) or just put blobs of MIG on there. And yes you CAN still file it, it's soft enough..
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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.