If the heads were cut .100", that's close to 20cc on 906's.
If you buy new heads, I would cc the old ones before deciding on exactly what to do with the new ones.
If you're really at 12:1+ now, I would be hesitant to lose much of that if added performance is the goal.
The 60' times are kinda soft for a 12:1 440 with a high TQ cam and 4.30 gears.
I suspect the "unknown" 10" converter may not be getting it done for you.
I've always been a believer in running as much stall as a combo can take( race car, not street car), and I'd be looking at like 4700-5000 with that combo....... But honestly, when I put new heads on I'd probably add about 10deg of duration and then up stall speed to 52-5300.