That's a real nice looking R Model you have there. Endt 866 V8 , 866 cubic inch displacement 375HP at the ground if it's tuned up and running right it should already be around 425hp at the flywheel. I ran one daily until about 4 yrs ago, it would walk alover a 400 Cummins especially loaded an on a hill. I would be careful about the modifcations I made to one of those be cause most of the parts for those are extinct now, and those you might find still on the odd shelf are extremely expensive. I still have a lot of parts for those here, but I don't have any literature on them, I too did the trial and error thing with the one I had. I had mine tuned at Weingand Mack in Sterling Heights, Michigan, but Ted the service manager that knew Endt V8s like the back of his hand has retired. If your pump has never been messed with or off I would say leave the timming alone. 45 mph is that uphill and loaded or on flat ground and what kind of weight are you pulling, what's the gearing in it? I would say if it is geared like a truck and not a race car it should have no problem doing what you want it to, it's probably somthing simple wrong with an easy fix, are you keeping your RPMs up when working it they don't like to be lugged too much, it's hard on the bearings and they don't like to sit and idle for long periods of time. It would be a shame to get rid of it if it is such low milage and a family airloom like that. It looks to be in beautiful shape Good luck with it, and keep trying you'll get it figured out eventually.