It all depends on how low you put the body over the frame. I've seen a lot of attempts at using an S-10 chassis under cars, sedan deliveries, and panels that have failed and the guy has given up on.
The kickup on the S-10 is very large and starts just behind the cab and the frame is that height the whole way to the rear. My '46 and my dad's '52 both have only about 12" of box floor (about 4" or 5" less than stock since we raised the bed floor and left the bed in the stock location. On a panel this can really mess up the floor.
It's your project though, your best bet is to do tons of measuring and comparing before you decide to go ahead.
Since you've already got the s-10 frame put it beside the panel and do lots of comparing and measuring. The kits are way overpriced for what you get and are best if you're not good with fab. However, since you're doing this on a panel you're best off to go a different route if you're scared of fab (which it doesn't sound like you are).
The worst part about the kits (besides the price) is they put the body too far above the frame for my liking. To each their own though and some people like the finished look.
The best method to do the body swap onto an S-10 frame is to have your engine mounted in the correct location first, then drop the cab (or entire body in your case) on the frame with the front fenders, grille, etc, attached and then take it off, trim accordingly, and repeat. On my cab we put it on and off probably 7 or 8 times before we got everything trimmed and the cab sat where we wanted it to. Because yours is a panel this would make it much harder. Just my opinion though. Hope it turns out the way you want, which ever way you go.
