I doubt you'll find a complete manifolds back kit for the 4.3l. Actually doubtful back in the day too.
My recommendation to you? Pick an acceptable exhaust setup for what your engine plans are in the future. If your car has overdrive, and you're comfortable building a 200-4r for it, go ahead and buy a dual exhaust crossmember for it. Then it's just a case of fabbing up something that connects to that for now, and getting use out of it after your swap. Wouldn't be too hard, a 4.3 is just a 350 with 2 cylinders lopped off design wise.
Of you search the forum, some members here have had issues with tailpipe bends on some cat-back kits out there. So do research on that.
I'd try to find a good local exhaust shop that will mandrel bend you some stainless pipes and install mufflers you bring them. That's the best fit for the body/undercarriage.
Or, if you just want cheap and disposable, buy it piece by piece from a parts chain and do stock replacement with used manifolds off something someone is parting out.