Thanks for your quick answer. I don't think it's room for the reducer.
But what about these that I just found on Summit Racings homepage:
Summit Racing® Heater Hose Adapters SUM-390007 - SummitRacing.com
Heater hose adapters. But I don't understand from the photo how they work...
Do you simply put the 5/8" hose into the 3/4" hose and clamp the connection?? Will that be leak free?
If so, I might have enough space for this.