273 302 318 340 build

The 1266C has 4 valve reliefs; it is not a flat top. It crosses over to the Sealed Power 285. So you will add 6-8 cc's of valve relief volume in to the equation and go back downwards on CR.

Stick with the 526AP's, IMHO.

Silvolite catalog is here; look at the top of page 22 for the 1226 valve relief pattern:
https://www.uempistons.com/catalogs/silvolite_catalog.pdf

That weight range of 778-805 grams that you were given does not make sense...... Even adding 161 pin weight to 592 gives 753 grams... Typical ring pack weight would be around 58 grams and adding that in does make sense either. Maybe they are including the carton weight? LOL