i've allready been wrong once in the last 24 hours about YT heads but i'm more confident in saying that i'm pretty sure they didn't do a grey head, a tell tale sign of a YT head is that the bottom of the valves have YT on them and these ones don't, although it is clear to see that the head is modified
sometimes yor just better off shitting in yor hands and clapping
Yep, Yella Terra did modify grey heads in the '70's ( maybe earlier ?? ) as I remember seeing Yella Terra modified head ads in car mags for grey motors that were around in the 70's.
From memory they had 3 stages.
Red motor valves.
+Port relief work
+ Larger valves ( maybe chev ?? ) with combustion chamber relief ( probably used in conjunction with dome top pistons I'd imagine ?? )