Looking over your specs . the 340 piston has about a 11 cc dish vs the 8.5 piston at 24.the 340 is also .030 taller then the 8.5 piston.
originally I wanted 9:1 but when I found out they didn't make 9:1 pistons, I decided to go 10:1. At this point, I'm not going to change it. The original goal here was a factory stock engine anyway. For fun I put my numbers into the CR calculator using a .020" head gasket and it comes out to 9.5:1 CR. When I did my first build in 1988 I'm pretty sure I used Fel-Pro gaskets and I can only guess they weren't .020". I used 10.25:1 Badger cast pistons but I didn't measure anything back then. All I know is the engine didn't like 87 octane at all and I always ran 92 or 93. If I can run 87 this time, I'll be a happy camper.
We hope your build works out for you, Andy. After you reinstall the engine, be sure the vacuum and centrifugal advance are working well with at least 46 to upwards of 52 degrees before TDC at cruising speed and rpms. For comparison sake, our old 1970 family sedan LeSabre with the 350 deuce got about 13.5 mpg overall and 16 mpg on long highway trips with the timing advanced using seat of the pants tuning. We did not have a timing light, so Dad and I kept advancing the timing and road testing until it pinged or until it was hard to start when hot. The BSB is certainly an under rated engine. They are very reliable and smooth, but not so good at gas mileage. For instance, during one winter morning back in the early 1980's in northern Minnesota, my sister unplugged the in-block heater by accident to the old Sabre during the previous evening. When my dad woke awoke to unplug the in-block heater the next morning, he bellowed, "who unplugged the f****** Buick!" Well it was 37 below that morning reported from the local radio station and the old girl was cold and stiff. Dad had to drive 40 miles to work that day so he stomped the gas to the floor, turned the key, and with the starter turning oh so slowly, she fired right up with perhaps a miss or two.
