You won't find an SM465 in a half ton... 3/4 tons and bigger only... another bomb proff tranny.
The GM 14-bolt full floater is just that... a full floating axle. The axle shafts don't bear any of the weight... the axle tube does... Well technically it has a hub and a spindle and that is what bears the weight. You can actually remove the axleshaft without even removing the wheel from the truck. Only if it has stock hub caps on it though and not mag wheels where the center cap is non-removeable.
The 10-bolt is... well... a 10-bolt. A weak rear end. IMO they never ever should have found their way under a truck. TH350 is a good tranny but it has it's limitations. If it has A TH350... that means it prolly has a NP203 transfer case. If that is the case... it prolly has full time four wheel drive too... Maybe not. Some of the trucks in 1981 were getting auto locking hubs but they were garbage. If it is full time four wheel drive... you will hate it. Eats a lot of gas and is hell on front end parts. There are conversion kits... but I think they aren't the best option.
Gary
To add to Gary's post, a GM 14 bolt refers to the number of bolts holding the ring gear on, (as does 10 bolt or 12 bolt, which coincidentally often is the number of bolts holding the rear cover on). There were two versions, the full floater and the semi floater.
Here's a full floater. It's an Eaton, but it looks pretty much the same as a 14 bolt. Notice how there's a protrusion that sticks through the wheel with 8 bolts on it. Unbolt those and the whole axleshaft pulls out. The axle has splines on the end of it that fit into the third member. The axle actually puts it's driving force on the outside of the hub/brakedrum. The axleshaft only transfers power. The hub/brakedrum is supported with two tapered roller bearings on each side with two nuts that preload the bearings. Lots of load capacity. If you break an axleshaft, your wheel doesn't fall off.
Another pic
By comparison, here's what they called the semi floater. Notice there is no protrusion? Absence of the 8 bolts too. Usually these type of shafts are held in with C-clips accessible by removing the cover. Bad. If a C-clip breaks, the axleshaft wanders out. Bad. The axleshaft carries the load and transfers the power. Bad. There's a roller bearing that rides on the outside edge of the axleshaft and that's your load carrier. Bad. This is bad rear for a pickup that's going to get used. Not as much bearing as a full floater and if you break an axleshaft, you lose the wheel. Also, when the axleshaft wears from the bearing riding on it, the axleshaft is junk. These rearends also had a smaller ring gear than a full floater. Most RWD cars, 1/2 ton pickups and some later 3/4 ton pickups have this. The year you're looking at may not have had this available, but be aware of it. Look for that protrusion through the wheel.
If you can find a 3/4 ton with a 350, Turbo 350 and a NP205 you've got it made. A 205 is going to have a cast iron case. Pics:
The stick