I don't know if you have figured this out yet...but thought I would throw in my $.015 CDN worth.

I use the Wallace Racing site, seems to have good data and lots of it.

http://www.wallaceracing.com/head1.htm

The 15/215 heads are 10.5:1 on a 400 with 1.96/1.66 valves. The YE block in 68 is a 340HP 400. There is a way to tell if you really have a 68 block in there and that is the casting number and date. I can't recall where it is on the block but should be pretty easy to find. My 68 Firebird has a YE block in it but it's from a 1969 and that makes it a 350 265HP. Your 1968 YE should have had 16 heads on it and 066 cam according to that site.

Good luck.