Comp cams xe268h for 318

BTW, you asked about lower CR's for trucks. IIRC those are listed in the Silvolite catalog; you can see the lower compression heights listed.

Trucks spend most all of their operating time at low RPM's near the peak of the torque curve with small cams. The small cams increase the DCR at low RPM's to optimize this. But the RPM being around the torque peak puts you the closest to detonation.... add in some poor octane fuel from a fuel pump in the 60's in the middle of no-where and it wants to go 'ka-boom'. So I presume that is why the CR was lowered in those uses.