A work Truck should get decent Mileage so it does not make you Broke driving weekly to work & back on errands.
Ideally you want 28-32.
Only going to happen with a 4 cylinder engine in a pickup Average in Town & highway use.
Towing uses lots of gasoline. Gets way worse with large cubic inch engines.
Looks like not an unlimited budget to spend.
TH 350 trans original in a 350 ci Chevy C10.
400 ci Chevy small block 4x4 had the TH400 trans.
The Lowest cost way & get best fuel economy on a tight budget of $1500-$2 K total to spend and absolute no more to spend is rebuild the 350 Chevy.
Bore it out to 355 ci.
Good valvejob.
Use cast Silvolite pistons.
Upgrade is Keith Black Hyperuentic pistons.
Otherwise it will start becoming a Hotrod project.
Before you know its a $20K- 50 K deal.
Flat tappet cam needed means he has very little $.