OK, so you found the limit of timing on the X-heads. That's what you have to do.... I did similarly with a 10.3 SCR/8.3 DCR iron headed engine with quench on 93 octane.
If your X-heads were at 66 cc's then you are only going up about 0.5-0.6 points on the SCR/DCR, not a full point. With that, the AL plus some quench/squish effect plus the small chambers will make up for than much and a bit more IMHO. So you have a good chance to be better off.
BTW, I get around 8.5 for DCR.
I think I'd time the cam if you can and push the cam timing back a couple of degrees. You're gonna pick up some low RPM torque with the added SCR/DCR.
