What is wrong with your engine?
It may be that it's rebuildable. Things like burning oil, low compression or low oil pressure may require a cylinder boring, new pistons/rings, cam/cam bearings and bottom end bearings. Worse case is the crankshaft needs turning, which is no big deal. Heads can be remachined as well, and new stem seals replaced. Basically a remanufactured engine. It's just not going to be inexpensive. You have to find the right shop you can drop off your long block and they do all of the work for you.
I'm assuming you want to keep the truck stock as it came from the factory?
Summit does seem to have a bunch of SBC engines for sale, but those are meant for like older vehicles with carburators. I don't see any Vortec-SBCs longblocks for sale. I do see some SBC bare blocks with provisions for roller cams and 1 piece rear main seal, which I believe should work with your Vortec setup. SUM-150203 for $1949.99. Just swap intakes from your current engine and whatnot.
Speaking of which, you need a budget if you haven't thought of one.