Finally got the 751 up on the roof.
Made only seemly marginal improvement, but channel 17, (RF 8), which is the one I was targeting, does seem to be more stable with less drops and pixilation. My Samsung TV shows 2 to 3 bars of signal strength with 18 to 19 SNR.
So I'm declaring success.
(I've noticed that anything less than 18 SNR is where I get pixilation and drop outs.)
To recap;
Had a DIY 10x9.5 Mclapp which is doing great, but weak on the VHF band. And the repack is moving more channels to VHF.
So I added a 751e with a AD combiner. The DIY 4 bay is now UHF only and the 751e is VHF only.
The fact that the Mclapp 4 bay received VHF channel 8 from about 60 miles away at all testifies to it's great design.