Using a lathe to turn down a piston crown is a bad idea. tell me how do I know? In the seventies when I was a lad I used a guy that had a lathe and was a machinist to turn down a set of 340 pistons for a 360 but he broke one piston and the rest were all over the place.I eventually got those pistons to work with a new piston and further machined by my engine specialist that I still know to this day. He used a special adapter piece to hold the bottom of the piston solid and milled them. I later sent him some pistons for my 340 that were 13to one forged pistons. they were obviously too high of comp for street use so I took him one 10.5 to one 340 pistons and told him to cut the other ones down to about 11 to 1 . He laughed and said he would give it a go. At the time I was a green horned twenty year old but now as a older gent in my late sixties I have learnt a lot about my younger years mistakes. That 340 ran great and did low thirteens back then on not so good tyres.
Obviously your well off in the cash departmenti value my time so why would i play around with some cockamamie bullshit like that? i'd rather pay the money and have the car back on the road and enjoying it than trying to squeeze and nickle and get six pennies.
this is also discounting the possibility of damaging the engine, or screwing up the job-- thus doing the work twice, and probably having to buy the components you screwed up in the process.
but, obviously, your brain isn't wired that way. which, again, why this is just so damn intriguing. carry on with the chicanery.