Hello 68 C20 and welcome to the Stovebolt
The manifold CAN be welded if it is not too badly fatigued or cracked. There are other manifolds out there to be had, may want to place an add in the swap-meet here on the stovebolt.
The manifold assembly is not too hard to replace but I do recommend having it resurfaced (machined for a proper fit to the head surface to avoid leaks.
I think you may find that part of the carb problem is from the manifold cracks/leaks.The intake/exhaust manifold shold be found as an assembly.
The 292 will do well for work but not get the same mileage as you could with a good 350. the 350 will be cheaper to work on as well. You will need to find engine mounts from a v-8 rig and a few pieces of linkage and an exhause system, The rest will bolt right in
