and the heavier duty super beetle mainshaft.
No such thing as a "Super Beetle" mainshaft. All SSC (Single Side Cover) transaxles had the same late model mainshaft. So for 73-79 all Beetles and Super Beetles had SSC transaxles woth the same mainshaft and 3.875 r/p.
To the original question... the SSC is not an option since you car is swingaxle and the SSC is IRS.
Do you plan on rebuilding it? If so there's a number of things you can do. Either way you want to reduce and eliminate wheel hop which can cause a broken tranny case.