The C6 will last forever, the only way you'll get a decent E4OD is if you buy one from BTS. Use a C6 and put a gear vendors behind it. There were 3 on ebay less than a month ago for good prices and they had the adapter plate to bolt up to the 1356 transfer case.
I have a good C6 with a 1345TC, well its my dads, but he'd be interested in parting with it if you want it. It's in one of our parts trucks right now, but we know the history on the truck and it was rebuilt only a month before it was totaled. Send me a PM if you're interested. You probably wouldnt need the 1345 since you already have a 1356 in your current E4OD setup, just reuse it on the C6.