IMHO go in the direction you were thinking of, first off talk to your machine shop about kits, here is an example of a budget kit there are many others:
Thats 190 bucks for all the bottom end stuff you need (short block). You have a good casting # block there to start with so have the shop check the crank, rods and block to make sure its rebuildable, if not then just buy a budget short block complete. Have your heads rebuilt and use them on your basically bone stock 350.,,,,OR:
#1 get your machinist on board and get a good one,,,
Buy these pistons with a 0.030 over bore
Cam just an example of a good towing type cam:
Now look for some vortec heads, search this sight for 500+ threads that refer to these excellent heads, maybe 500 or so bucks for a complete set ready to go depending on the cam lift, a good RV cam will most likely be OK with stock vortec head as the lift will be below .450 and duration should be around 208 to 212. These heads are 64cc so make sure to get the dished pistons not flat tops to keep the DCR at about 9:1 this is were your machinist comes in cuase he will need to use compession height of pistons to determine deck height when machining block to end up with about 0.040 -0.045 squish with a 0.040 head gasket.
Intake,,,edelbrock spread bore, 175 bucks, suggest this one for factory Qjet carb, you may find carb easy for about 325 new or 125 rebuilt, go with Qjet on your build if you want to get some good mileage and still have good WOT pulls, maybe you have this Qjet already on the 350?. Use felpro MS98000T intake gasket kit they are expensive but you may just be able to buy the intake gasket (2) if you know your parts guy, these do a better job of sealing up the vortec heads to intake and can be reused.
can't beat this value
Oil pan, timing cover, valve covers, plugs/wires, balancer,fuel pump another $400 maybe or search the junk yards for the covers and pan to save$$$
Add some headers if in the budget and some dynomax turbo mufflers another 800-900 maybe other wise just good dual system with the same mufflers, but headers would be best for sure.
You will have about 325HP and 375ft/lbs on a completely driver freindly truck that you can use for your towing and have some fun while doing it on regular gas. There are some that will have you building a street/stip motor before you know it, don't go there if this is a work horse.