my Fuel injected 88 gets about 10.5 towing at 65mph. That was a 5600lb load of a Toyota truck on trailer. best ever was just truck and it was 11.5. My wife DD's it and it gets 8.6. That's short hops to the grocery store no hwy.
The truck is R20 (3/4 ton) 454 with TH400 and the Full floater 14 bolt with 3.73 rear end. Other then brake size that it the same axle the 1 ton single wheel trucks got.
Go with the Fuel injection. The mid 80's Carbs are a pain. If your going car I would say go older like 82 ond older. In 91 the 4L80e was available. Those would pull 13.5 empty.
These engines are low RPM. The fuel injected motors make 385lbft of torque at 1600 rpm
They will pull a house off it's foundation.
They DO NOT like high rpm. their sweet spot ends at 2500 RPM (about 65mph). Over that they start using a lot of gas and running warm. I am a gaget freak and I have added temp gages to both transmission and engine oil. At steady 65 mph with cruise control on and 5600lb of trailer behind it it will run all day long in 85degree ambiant temp with the A/C on and loose on 1.5 mph climbing most hills. The Engine will run 195 degrr coolant, oil temp will be 210-215 and tranny temp will be about 185.
At 3,000 rpm EMPTY it will run 200 degrees engine, Engine oil will hit 290 degrees and the tranny will run 200-210. Exit the hwy and stop on the ramp and the engine will come up to 210-215 the AUX cooling fan is on and it will take it 3-4 miles of low speed driving to come back down.
Truck has 84k on it, new radiator and extra Tranny cooler.
Only other thing to warn you about is make no mistake. It's a TRUCK. The 1/2 tons ride pretty cushy but the 3/4s are stiff. 500lb of tounge weight on the hitch only cause the rear to loose 1 inch of ride hight and the front to come up 1/2 an inch. With my WD hitch hooked up the truck rides 1/4 inch low in the rear and stock front. Have kidney belt ready for rough roads. LOL