flat tappet cam? no no no no no no. its not 1989.
Amsoil makes 10/30 in their z-rod line. But with a roller cam it isn't really needed.
I have roughly 14.5" at idle and have no power brake issues.
Edit: above link is beat, i think it is this one. That says roller lifters.
http://nyesautomotive.net/406_525hp_EFI.html
"Erson hydraulic roller cam and lifters"
"This engine is fully machined, balanced, assembled, and
dyno tested."
If the builder says 40 or 50 weight, he should know, he set the bearing clearances.