The TargetMaster is a good motor for a relatively stock one. But it should provide plenty of power for your truck. Just my own opinion, but I'd go with an Edelbrock 1406 carb and 2101 manifold if you're gonna go with the stock TargetMaster cam. Good bottom end, easy to install and "set it and forget it" adjustments on the carb. Well..unless ya like to fool with things.
I'm not sure what you mean by TH350 with overdrive, unless you mean adding a Gear Vendors' OD unit behind the TH350. If so, this ain't a bolt-in operation, nor is it an inexpensive way to go. The GV OD runs around $2k.
There are basically 2 OD transmissions that can be intsalled relatively easily: the TH200-4R and TH700R4. The choice between the 2 is a matter of great debate, but my preference is the TH200-4R. I like the gear spacing better and it's the same length as a TH350 or Powerglide, where the TH700 is 3.5 inches longer.
I don't have enough experience on the rear ends on the AD trucks...but if you go with a TH350, the stick rear end will give the same engine speed as the current trans, since both are 1:1 high gear setups. If you want to drop the rpms for cruising, then an overdrive or rear end (or gear) swap is called for. Or you could do like I do with my 64 (4:10 gears) and drive in the slow lane and wave to people as they pass.
Best of luck with it
