Someone correct me if I'm wrong, but my understanding is that the heat riser was strictly to aid in warm up for quicker driveability.
As I mentioned, I never noticed the difference on my trucks. I had a 69 six and a 68 six and both had the heat riser removed at some time or another.
I suppose that if you just bloc it off you should be okay. Maybe a few minutes of warmup in the winter (which most people do anyway).
Clifford Performance sells the water heated intake manifold which I suppose takes the place of the heat riser. Offenhauser sells these parts also. Check them out. May even want to call them to get their opinion. These guys have been in the business for a long time.
http://www.cliffordperformance.net/
http://www.merchantamerica.com/chevy...&product=75243