Radiator:
If your original radiator is still in place (and it was performing/cooling well other than the leak), measure it and use those dimensions when ordering a new one. The biggest hassle when replacing a radiator is often fitting it in place. This is due to sloppy, "one size fits all" aftermarket manufacturing processes. For example, do you have A/C? If so, the radiator is a couple of inches taller.
When selecting a replacement radiator, order according to model/year of the truck and the size of the current engine. IOW - you should order a radiator for a 1981 K20 with a 5.7 liter engine (also with automatic transmission and equipped with A/C - if that's the case).
Best bet on replacement radiator is - stay away from the internet, buy local. Bring your current radiator with you to the parts counter and check all the dimensions carefully to make sure it will fit.
Belts:
To help you with part numbers on this, you would have to provide a few images of how your engine and auxiliaries are set up.
But, basically, if the current belts work well, use the same sizes. Remove what you have installed, take them to your local part store and buy exact replacements. Or if you prefer, get the sizes/styles off them and order on-line.
Hoses:
Not counting the radiator/expansion tank vent line (and heater hoses) there are only two hoses - upper and lower radiator. And, other than getting a lower hose with an anti-collapse spring, there aren't many options. Get any of these at your local parts store:
Upper: GOODYEAR Part # 61564, DAYCO Part # 71558, GATES Part # 21204
Lower: GOODYEAR Part # 60855, DAYCO Part # 70735 , GATES Part # 20843