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What's the chassis number of your J-H? According to Richard Calver, J-H's with chassis numbers between approximately 18300 and 18900 had the federal rubber bumpers (making them look like J-H5s) but the 4-speed drive train. Is it possible your J-H is one of these vehicles, and a PO upgraded the gearbox to the Getrag at some later stage? It may even be possible that your J-H was built in its current configuration, checking the build sheet (which you can get from Martin Robey here in the UK) would confirm.
I don't know about the States, but here in the UK where thousands of Vauxhall Magnum 2300s (as well as a few hundred J-H5s and GTs) fitted with the 3.45:1 cwp axle were sold, this axle is now extremely rare. I managed to find a spare one for my GT from a Vauxhall racer, which is now being fitted with an LSD. In the States you may struggle to find one unless someone is breaking a J-H5 or GT.
You may be better off keeping your 3.73:1 cwp axle and swapping out the now very rare Getrag for the easy to find Toyota W58 gearbox, which is a common conversion giving a proper overdrive 5th gear.
Apparently with a stock 907 engine the W58 is a good match for the 3.73:1 cwp axle.
Hope this helps.