I was thinking the 1:66 twist because its close to the Pedersoli (1:65) & GM (1:70), and same as Lyman GPR.
Bob Hoyt can put any twist in it and recommended the 1:60 for the .54. I did a couple searches for the optimum twist rate for shooting PRBs in a .54 but couldn't find a solid answer. If I go this route I may just take the expert's advice and go with the 1:60 unless someone can prove to me the 66 is better. Keep the comments coming.