The HP manifolds I have are -
RH 2899879 N4
LH 2951865-2 B7C
Made on dayshift 22nd October 1969

The testing I have done so far has proved the LH manifold is the better of the two

Runner #5 & #7 are close
#1 is the best flowing, but not a heap better than #5 & #7
#3 has 40% more restriction/back pressure than the others

The RH manifold is worse.
Runner #8 is the best of both sides, showing 1/2 the restriction/back pressure of #3
#4 is a bit better than #1, #5 & #7
while #2 is a bit worse than #1, #5 & #7
#6 is the worst of all & shows 120% more restriction/back pressure than #8

If I can get my flow issues sorted out I'm going all the way with anti-reversion at both joints (heads to manifolds & manifolds to headpipes), thermal barrier coatings on manifolds & headpipes etc

I'm running 2 1/2" pipes now but I have started to build a 3" to 2 1/2" antireversion fitting at the flange between the manifold & headpipe. As there is documentation that a 3" headpipe helps the HP manifolds work.