The stock 3 speed had no support, it was just held on by the bolts on the bellhousing. The stock crossmember supported the bellhousing, so no it won't work for your 700r4.
If you are going to be running an underfloor master cylinder make sure you get that mocked up first because you will need to work around it for the transmission crossmember.
Alot of guys have used a universal crossmember like this one. Check out KMS Tools, Scotts Supertrucks in Penhold or Bills Truck Shop for Canadian retailers
https://www.kmstools.com/cpp-1947-59...smember-112541
https://www.kmstools.com/counterpart...ed-or-pg-14971
Another place to look if you want to fab something up is Welder Series in Ontario. I've used their transmission crossmember and 4 link in my 59.
Good luck on your project!