hi scott, you need to go to mikeanics in congleton i took my spitfire there on the recommendation of a friend of mine who has taken his before he knows his stuff when it comes to our engines http://jc-webs.co.uk/mikeanics/index.htm andy
all i was thinking was that it is getting effectively unmetered air from the other carb, so i spose you would just have to richen the mixture a bit to compensate. still reckon it is best to seal them off
i have a spitfire with a 2.5 engine and webers, i kept the original 1500 heater valve location. the rear end of the head has a pipe going to the heater valve, then the out of the heater goes straight into the pipe under the manifolds to the pump.
150s are 1.5 inch and 175 are 1.75 inch, which are the same as the hs6's. i would go straight for the hs6's if you need any more advice give us a shout i did this conversion a few years ago