the later combination style exhaust manifold is supposed to to the pre-heating function. It works okay, but something closer to the later volvo setup with a steel shell around the manifold that leads to the air filter might work better.

I was using a pair of 140-style HIF6s that had a 3" hose on the air filter box that went down towards the manifold, so I'd guess that there is a 140 manifold shell that might be just what you need.

Then again, I may have mis-interpreted the function of that hose...