Compression reading alone won't tell you that, and $395 is wayyy to cheap to have a set of heads rebuilt. $395 each is probably within reason. But you need a leakdown test to determin, valves or rings, or head gasket. Comp test tells you there is a problem, leakdown tells you where it is
A truck 302 out of a 90's F series is in the realm of a $300 motor running. It will either be roller cam ready or have a roller cam ( 1993 and later. I think. don't quote me on that ). That only matters for the distributor because you would need to swap distributor gears on your marine distributor if it were a roller cam.. You just need to put marine core plugs and head gaskets into it and possibly change front cover and oil pan depending, since Ford used a number of different things. You can check the compression on a motor even before you buy it
You could also do a wet compression test.
Squirt some oil in the cylinder and take a reading.
If the reading is the same as a dry test the rings are good. Pointing to a head problem.
If the reading is higher the rings are bad.
From your compression test readings of 120 - 140 something is wrong. Shouldn't really be more than 10 difference.
Oil on plugs, my guess worn oil rings on pistons.
If you do rebuild the motor make sure they check the heads and block decks for even/flatness.