Different heat range as well , and 2 types fitted to the P6 lump to cure a misfire but I can never remember which is which :down: Think it's L92Y cures the misfire and th original plug is an L88Y. Anyway SD1 etc heads use a RN9Y .The spark plugs on the 10.5 comp engine only have a short thread in the heads so be careful not to strip the threads when tightening the plugs....The exhaust ports are also quite restrictive - you can put later heads on with bigger valves.
Rimmer Bros stock the electric choke parts, as I'm sure other Holley carb dealers do..possibly find them on eBay too.i thought it was a manual choke and i got told it it was is there any way of changing it to a electronic choke
SD1 or 3.9 heads will give you bigger valves and marginally better ports, and so a useful power increase coupled with the 10.5:1 pistons.also I'm thinking about getting a set of 3.9 pistons and the head of a 3.9 like you said for the bigger valves
(i know it will need boring for this. But does this sound like a good idea or am i just dreaming sat here with my tea)