The Probe SRS 428 pistons are forged with a flat top with 2 valve reliefs (total volume of 5.5ccs) , sorry I didn't mention that.
To avoid boring your 389 block excessively you can get Ross forged pistons in pretty much any bore size as long as there is a piston ring set available in the size you want. To do it a bit more affordably you'll have to settle for 428 pistons in either std or +.005", unfortunately std bore 428 pistons are not readily available off the shelf these days.
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