Here you go, it IS the pistons. I've had my hands on these back in the day. Driven a 303 HP 389 where the guy put in the dished pistons and it was tough to tell much of a difference in performance. You might get away with 89 with these guys...maybe..
https://www.ebay.com/itm/25491660366...UAAOSwUuFW10Hm
The School me on 389's guy might be interested in these as well.