I had a '90 2500 Chevy with the 350 TBI engine and 4L80 tranny 4.10 gears. That particular model 350 in the C6P chassis made its peak torque at 2400 rpm. Hp/torque was 210/300 . I think the later model TBI engine had a bit more power. The truck averaged 9-10 mpg pulling a 6700 lb 5th wheel trailer and a 3400 lb bass boat behind the trailer. This was in the 55 mph max speed limit era. It averaged 17-17.5 mpg running with a empty truck on long trips. Never had a city for that kind of millage.
My wife has a '06 1500 5.3 chevy crew cab Z71 package 3.73 4L60E tranny 295 hp and averages around 17-18 on long trips running empty. I use it pulling my blue tractor on a 10k car hauler with a 7500 to 9500 lbs gross depending on how much equipment I take for a job. It averages a steady 9-10 on 150 mile trips to a job.
The old 350 has more grunt in the 2500-2800 rpm range for getting down the road and maintaining speed. Over 3200 rpm and its simply making noise.
The wifes '06 1500 5.3 needs lots of rpms to keep pace with the old 350. Sure once the 5.3 gets in the 3500-4000 rpm range then it will hide from the old 350 TBI engine but lordy does it get tiring hearing the noise from all that rpm.