"...My machinist used Keith Black Hypereuctectic dished pistons. They have a 16-17cc dish and the valve pockets will work with the 389 heads. Take a look at Spotts Performance web site. They offer them for $339 a set unless the price just increased. The part number is KB346..."
They're $384 + shipping at CNC. So, that Spotts price may be old. But, if it's current, its the best price I know of.
https://www.cnc-motorsports.com/kb346-std-keith-black-pistons-dish-4120-bore.html
I wouldn't use the KB Hypers. Lots of guys have had piston failure, with those. The problem seems to be with the top ring. Unless you go with a huge end gap, the ends will butt together after the pistons warm up. Also, they are cast--not forged.
If you go with a 4.120 or bigger piston, there are stronger forged piston options.
I mentioned the SP L2262F pistons, with some material removed from the center, to reduce CR. I think this might be my choice, since I've had good luck with all the SP/TRW forged pistons I've raced.
https://www.ebay.com/itm/Speed-Pro-...h=item27fc4c8a94:g:-BYAAOSwG-1Wvh9s:rk:1:pf:0
I also mentioned the KB Icon forged pistons, which have 8 valve reliefs. These will probably work, with some material removed from the valve relief area.
https://www.summitracing.com/parts/uem-ic9946-std/overview/make/pontiac
Again, if current, the Spotts price is cheaper, @ $399 + shipping.
http://www.spottsperformance.com/Pistons
Then there are the SP L2323F 4.125 bore Olds 455 pistons. Here's a set for $250. Will need to take a little off the top, to achieve zero deck height. They already have a .142 dish. So, machining the tops should be easy.
https://www.ebay.com/itm/Speed-Pro-...h=item56bf93ce1a:g:LdcAAOSwgotcTk0Z:rk:1:pf:0
I suppose you could lower the pistons in the holes, a slight amount, by offset grinding the crank, for a slightly shorter stroke. Don't know if that costs more than a regular crank grinding or not. The less the crank is worn, the more offset can be ground in. Might need to cut to a .030 undersize to get any significant stroke reduction. But, if you ever need another crank, I assume that most any Pontiac 350, 389, or 400 crank can be used. So, a good core crank should be real cheap. Lots of guys have tried to give Pontiac 350's away & couldn't. And, there are lots of extra 400 cranks, because of all the 400 block stroker builds, & because of so many cracked or otherwise unusable blocks.