No my holly books say 1.5 to 2.5 below in gear idle vacuum so a 6.5 would likely be spot on.

you use 2.5's om 300 @ .050 roller cams!!

You must have misunderstood or the 3.75 a hour kids are confusing you!
Thrust me try a 6.5


the 1/2 vacuum numbers are for real stock cams with 17 in gear vacuum so 1/2 is 8.5

Power valve is $9 and won't hurt anything (you probaly have a 6.5 laying around) quick and easy to try an cannot hurt anything. If it was a 8.5 with 9: vaccum it might flutter open at idle and be a bit rich but a 7.5 and down will be just fine.