There are no set rules for which plug or heat range of plug that are best under all conditions. It depends on fuel. atmospheric conditions etc. but perhaps this quote from E-bay could give you a general guide:

"Use a hot plug when using fuel with a low nitro content (less than 24%), and a cold plug when using fuel with a high nitro content (25% and higher)

If you remove your glow plug heater from your idling engine, and there's a noticibly large drop in RPM, you may need a hotter plug or a fuel with a higher nitro content.

If your engine tends to backfire often, you may be using a glow plug which is too hot, or you may need to use a fuel with a lower nitro content."

Whether or not yo agree with that information, you really need to try them out for yourself, to see which you think is better. Nearly all the experienced drivers where we race seem to think that plugs that are comparable to Nova 6 are usually the ideal; but then there is one guy who swears that Nova 5 is the ideal, and he has some of the fastest engines????????

The different manifolds are designed for bottom-end, mid-range or top-end performance. The track that you use the 2660 pipe on, should determine which manifold to use. The 2660 is supposed to be a great top end pipe, and we like using it on fast tracks with a 5 fin header.

Kindest regards,
Lars.