Bruce,
Even 5-10 psig less in one cylinder would be cause for concern as indicating
a burned valve or other serious compression leak. Your compression gauge and
method of measuring (e.g., plugs pulled/not-pulled, carb butterfly
open/closed) will make a big difference, so it's best to compare cylinder
variations rather than compare with the published value. 125 psig is
probably in the right ball park though. Squirt some oil in #4 and re-check
the compression. If it helps, that means it's rings. 100 psig does sound
like enough compression to fire at higher rpms, but if it's an intake valve
that is burned, the blow-by may disrupt the intake flow and cause a problem
with less compression leak than for an exhaust valve.
The Edelbrock Performer has bigger intakes than the 260 heads, which are
tiny. You could do some porting work on the 260 heads to get the runners to
match, but the Edelbrock Performer (RPM?) head is undoubtedly overkill for
the 260 heads.
Just a few thoughts,
Bob
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