Ray your on the right track measuring everything until you find that which is too tight
Here are some you have not posted
piston ring side clearance
piston ring back space
valve stem to guide
valve lifter to bore
camshaft journal to bore
and the list goes on but any of these can...
New seats will have inst5"ructions with sizes
generally depending on material of the seats 0.003"-0.005" interference for cast iron
I go 0.005"-0.006" in side valve engines
Larry s method works well and I have used it with original style valves and keepers
But if you have modern style valves and keepers it is easier to remove the head
Well designed tube headers should be equal length and of a diameter that makes for a scavenge effect that will give a decent power increase
length and diameter are normally the result of trial on a dyno
most of the off the shelf ones I have seen are not equal length so not much better then...