Vk valiant 142 weiand project

The drop in PSI and the MPH staying pretty much the same is proved to be a restriction of flow in the manifold.
How much PSI are you seeing now with the smaller pulley?
I wouldn't worry about over heating the intake charge. If you can't get high octane fuel, maybe add water/ meth injection.
Or if you want more power and want a really cool, dense intake charge, whack it with a 100 shot of Nitrous Oxide. Use a WOT micro switch on the carb for activation. Nitrous will be at -125* F and will add roughly 33% more oxygen.
Now might be a good time to ask, what head gaskets sre you using?