road race engine...

do the magnums you are talking about have 8mm or 5/16 stems
if so they are not using sbc valves
we used to use +100 sbc valves in LA motors with the conversion guides or 5/16
some backset the studs in magnums and run I think BBC rockers or maybe ford
this also gives you room for larger pushrods
problem with the stock magnums is they crack- mine did
The aftermarket al heads should have better ports otb than the stock magnums
and 1.6 exhaust is plenty for a road race motor
you need to keep your velocities up - you are not building a drag race motor
keep at it
who has the list of roller rocker arm lengths
btw the stock magnum rockers work fine, some say to quite high lifts