I have a 84 with a 351w and it pulls like a sob. I have towed a 5klb boat from sac to tahoe up 50 ( put it on cruise and all it did was gain a 250ish RPMs) now I towed a dump trailer with 2.5 yards( 7-7.5k total) of gravel up 50 and it did slow down, but only down to 45mph on the steeper grades. I would not want to pull it long distances do to the short wheel base. oh yeah I have pulled out alot of trucks out of the mud on the job site.
now gas milage on it... well its about 10 mpg on the hwy( if you go 60... if you go faster the milage will drop like a rock) or street... now towing milage for thos trips was between 5-7 mpg.
Thanks for the info Yellow...these 351's run strong!!