Fly cut

Say I were to get a Kasse, AFR or other canted valve style head & wanted NOT to have to buy pistons. Is fly cutting a piston in the engine with the head you are using legit, or trailer trash?

Well, it's kind of 'in between'. There's nothing wrong with doing it, as long as the area being cut is thick enough that there is still some structural strength to the piston, and that you dont take out so much material that it will change the weight (and the balance factor of the rotating assembly).
And you'll want to vacuum out the cylinders real good, and get every last piece of metal shaving or aluminum grit that is present.

It's fine, as long as the piston is thick enough to support it. You can't possibly take enough material out of the piston to affect the balance.

Most stock style pistons, particularly dish tops, are thicker than A-10 depleted uranium rounds.

A lindy tool is what you need.I used one of his cutters on a set of flat tops for the afr heads .You can rent this tool from Lindy also I believe he made my tool 2.300 for my 2.250 valves .It is a time consuming job and takes some patience but well worth the effort .Lindy is on the web .Doug