I would look (measure) at the pistons wrist pin location and how much differences there are in the rod length and then see if it is possible to figure out how much higher in the cylinders the pistons your going to use will be and se if that can help you figure how much more compression you will end up with.
Are you wanting to use auto gas in the motor?
I owned and flew a Piper Comanche 260 HP for 15 yrs and always ran 100 LL in it even though it was 8.5 to 1 compression ratio, I'm sure it would have ran fine on 86 or 88 octane autogas twocents
Stock ignition timing called for either 26 or 28 BTDC on both mags, me thinks I may have set mine at 29 or 30 BTDC whistling devil
I lived in SO CA and flew out of L26 which was 3100 FT on the low end of runway 23 and a little over 3250 Ft on the displaced threshold on runway 50, I always did a full throttle run up and adjusted the mixture to between 25F to 50F rich of peak before take off wrench up

Last edited by Cab_Burge; 03/27/20 10:57 PM.
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)