Valve stem seals

There are better seals
the neoprene fel pros end up in the pan unless you run stock revs and no long pulls up hills
check the truck listings
or the BBC HD umbrella seals if they fit inside SBM springs I've used these in BBM
there are both Silicone and Viton
other engines with 3/8 stems like FE fords
a problem with stock umbrella seals is they loose control with high rpm operation as well as high temp (center two exhausts)
what are the MP seals made of that mancini sells- the ones on the right
the next ones look like teflon seals- pass
the comp ones are suspicious
I do not see how you can have positive seals without cutting the guides
check out those mp seals
they are most likely availavle from multiple sources once you determine the materiel