There is one source that is just "a little more trick" than stock. Try www.performanceamericanstyle.com. They have a stock tube cut in half with brass fittings so you can remove the part just above the block easily to install headers and then reinstall after. I have used it and it works very well. Easier to install than the stock unit.
I have not seen a Lokar style tube for the AMC. Probably could fab one, but you would have to use the cut the tube in half technique, use the bottom half, and then make sure you have the correct length.