Were the X heads put on '72 340 dusters? If not, what advantages would there be if any? Any effect on compression ratio? Just curious because my '72 duster has them. Any input would be greatly appreciated. Thanks.
X heads were used on 68-70 340's only. They are nearly identical to the big valve J heads used in 71 but your 72 340 would have had small valve heads on it so they might be a small perf. improvement, very small. Reason is the 72 had a much milder cam and low compression that won't utilize the extra flow capability of the large valves.