Can't help you with the heater portion, because I don't run one in my 2276 in S. Cal. A 2276 is not too much motor for these cars IMHO. I'm running high lift narrow LSA Web86B cam, 1.25 ratio rockers and 9.5:1 compression, dual Weber 44s, LWFW and 4sp. I don't over heat or have any pinging on 91 octane. The engine came out of a sand-railer and I had a local VW outfit go through it before it went into the Speedster. The local builder BTW holds all if not most of the current 1/8, 1/4 mile aircooled VW records and so they know how to build a motor. I did swap out the Porsche OEM Fuchs with Porsche OEM Steel wheels and I'm happy that I have the extra HP because those steel wheels are HEAVY! I do mean what I said about a 1600 or even a 1915. A smaller, more peaky higher revving engine compared to my 2276 would change the character of the Speedster so much that it would fell like an entirely different car - in a good way.
If I had to do it over again? Maybe a Suby :ducking out of the way: