You can change the tail housing in the car - and fairly easy....drop the driveshaft, remove the speedo cable, take off the plate with two large screws, remove the bolts that hold on the tailshaft housing (5 I think?), spread the retaining ring under the plate you removed and the housing slides off. Watch for the fluid! If you post on here you should be able to find a tail housing for a fair price. I have one in western Canada I could throw in the mail... This is a great time to tweak the governor (located on the trans output shaft) if you are not happy with the shift points.
And yes, the cast lugs on the trans started to dissapear by the early to mid 70's