Prouzy, either of the 63 grain 257 molds should do fairly well . I wouldn’t bother with a smaller mold as you would be stuck if they were to drop too small. Ebay also has the odd .257 mold come up .
Sounds fun I have the Escape UL 18" .22 unchoked barrel making a solid 90 FPE with 50 gr. African Air Ordnance slugs. That's is with stock valves and an o-ring buffer on it still. Also have the stock .25 barrel I will have to swap out and test NOE 40 gr. BBT HP in yet.