The small block Chevy is lighter than the original 6, so that won't be an issue. The frame is more than strong enough to handle any V8 you would want throw in it, no boxing necessary, in fact, it could be detrimental. The frames on those trucks are designed to flex, that's what makes them as strong as they are. The only reason you might need to add any crossmembers would be to allow for engine/transmission mounting, depending on how you do it.
Brakes might be an issue, since you have a 3600. There really are no kits to convert to disk. However, if you don't mind a little fabrication, you might consider
this option. If you're also considering rear disk, check
this out.
On steering, there are going to be a few issues. With the stock steering box, the drivers side exhaust manifold will interfere with the box on a V8 swap. Some guys move the box over a bit, others do a power steering swap with either a later model GM box or an early '80s Toyota truck box. On the V8 swaps I was involved with, one truck got subframed with a Camaro sub, the other got the box moved over. Personally, I would look into a smaller steering box. There is a rack and pinion setup out there for an I beam, I have seen one on a local F100. From the standpoint of someone that does suspension and alignment work professionally, those setups look hokey at best.
