Is 100 though too much to take off a 295 head?

why ruin a rare head? Get high compression pistons, flat tops are available.

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The bitterness of poor quality (or Crappy Workmanship) remains LONG AFTER the Sweet Taste of Low Price is forgotten.

So 100 thou is going over the top? Will the head still be functional?

These days I would not take more than .075" off any early small bore head, including the 295. Max .060" off is safer.
Reasons:
1. They had a much thinner deck than 1275heads when new.
2. After 50 years of corrosion they are even thinner in there. Expect head gasket leaks if you rip too much off.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.