Actually there were quite a few intake manifolds made for the R5/P7 combination.
I have a few of each here, some of what I have are no longer NASCAR legal, I am not sure why but that is a fact so if you need pictures or info I will try to help.
We have built and dynoed about every combination of parts that are available for the R5-P7 (most in roller cam form) and to be honest I think the port work means more than the part number.
Certainly there are a few manifolds that have much larger volume than others, but as changes were made and time went on the runners became shorter. No doubt the intent was to move peak power up higher in the RPM range.
I have one motor here that makes peak at 9300, not that long ago this thing would have been a decent Comp Eliminator motor, think about it this way, it has a cast manifold, a circle track style cam, 12.5 Compression and was dynoed with a nothing carb.It makes almost 2.4 per cube with break in oil. A few modest changes and it can still be very good.
Actually we are having a few sheet manifolds made for these things, if you think you want to go down that road let us know.

Rick