I'd say it's a great choice of engine to build up for the work you're doing. If you're pulling wagons around the farm, hauling and such it's perfect. 15k is a lot of weight, the 300 will move it just not in a big hurry... I wouldn't suggest going on the highway with it. Most 3/4 ton pickups of those years weren't rated for that much weight. However running back country roads you should be fine if you take it easy. Truthfully a bone stock 300 six is about as tough and reliable as they come. Speaking from personal experience, they will take a tremendous amount of abuse. The one thing I have learned about these engines is gearing is a critical part of making them perform. IMO they work best with a manual transmission behind them, more specifically an NP435, BW T18, or T19. The 300 six coupled with an old cast iron granny 4 speed and 4.10 gears (especially in a 3/4 ton or heavier 4wd) is an unbeatable combination for farm work.
If you're doing a ground up rebuild I would start with a mild RV cam, it will give you better off-idle torque. Stock crank, rods, and pistons will suffice; go with good quality bearings and piston rings. For bearings I prefer Clevite, for rings I like either Hastings or Sealed Power... high volume oil pump is a must. You could do some mild P&P work on the head and gasket matching never hurts. One thing I noticed that really wakes up the 300 is breathing. Ditch the stock one piece single outlet exhaust manifold and either go with headers or a set of EFI manifolds, the EFI 300s used a 2 piece manifold set up it was a huge improvement over the old set up. For what you're doing I think I would go with the EFI manifolds over headers, simply because cast iron manifolds are more durable than headers. Another thing is to ditch the stock 1bbl carb for either a 2bbl or 4bbl, there are adapters to install a 2300 series Holley 2bbl on the stock intake. Some will argue that they don't help performance, I from personal experience say they do... the difference was night and day. The best way to go is with either a Clifford or Offenhauser DP aluminum intake and a 4bbl carb, but it's more expensive to set up. If you ask me inline sixes score really high on the cool factor. They're tough, torquey, and won't let you down.