Tom,

I don't think the SDL would be getting "pushed too hard" even if you were going 90 mph for an extended period. The engine speed would be about 3900 rpm which is fine for continuous use with the 603.

Now, try that with a 617 and you are looking at 4200 rpm. Now that is pushing the five cylinder quite a bit. Personally, I would not take the SD up to that speed.