Have you considered a "spool" instead of welded spiders? I have seen them offered in both aluminum and steel. I would use a steel.
The solid spools are very common, for regular differentials, nine inch Ford etc. At the time I was doing mine there was nothing available for front wheel drive transaxles. On mine, a 125 GM, you have to pull the trans all apart to reach the differential. On some of the four speed models the final drive can be separatedd from the main case which would make it much easier. I do not typically rebuild auto transmissions but in this case I was able to pull it off.