In theory, conversion to an old-school vacuum boost should be fairly straightforward, but there are many details that need to be worked out and some of the brake lines need to be reconfigured.
I don’t recall what the situation with the proportioning valve for the rear brakes was (it is also frequently discussed here).
Paging @BobPhx
He did the conversion (couple to a modification from auto to 5-speed manual) and documented the experience here:
https://forums.jag-lovers.com/t/teves-iii-convert-back-to-vacuum-booster-questions/