<VV> 65 Monza 110hp engine question

[Larry Forman]

Last August my wife and I went to Reno's Hot August Nights car auction 
and I drove away with a midnight blue 1965 Monza PG convertible and 
decided to make that my daily driver.   It was claimed to have only 
5,000 miles on an engine rebuild.   The oil was black and once changed 
to synthetic the valves quieted down and CHT gauge reading went down 
about 20F, still well under 350F .  The car and engine runs fine and I 
checked the engine compression and it was generally around 125 lbs. 
except for #4 that was about 100 lbs.  I still need to squirt some oil 
in there and see if rings are to blame or maybe a valve guide.

The spark plugs were Autolite 425s that are only 1/2" reach, so I 
changed them to Autolite 65s with the correct 3/4" reach and ... the 
engine immediately had a knock.   Removed the plugs and sure enough #6 
and #1 had the ground electrode hit by the piston top.   A couple of 
others had the piston top kiss the cylindrical end part of the spark plug.

Comments please!    I am assuming that someone might have "adjusted" the 
base gasket or somehow changed the head relative to the piston, perhaps 
by milling it to make it flat without compensating for the base 
gasket?   I am also concerned that the valve geometry is way off.   Any 
ideas what happened when the engine was rebuilt and what else I should 
be concerned about?

The good news is that it runs fine and makes a very fun daily driver.   
Positive comments nearly everywhere I go.   Certainly not what I was 
used to with my 2015 Honda Accord, which has now been sold without any 
regrets.

Thanks for any help.

Larry