When I swapped a 4-speed into my auto 1970 road runner, I used the same crossmember but turned it around backwards and it worked fine.
The trans crossmember on a 69 is different from a 4-speed to an automatic but as mentioned can be adapted by cutting out the center to let the mount go down in there. In 70 4-speed or automatic are the same crossmember that is providing you are using a 70 4-speed trans if you are using a 69 or earlier 4-speed trans it would be a different one for automatic or 4-speed. There are also differences in the bolt hole size in 70 crossmembers on the body and trans.
HI Moparparts- i just started a thread a few days ago on my auto to 4 speed conversion in my 70 RR. I have a 69 trans. From your post it sounds like my 70 auto trans crossmember won't work on my 69 4 speed. Can I just reverse it like it was mentioned in this thread or do I have to get a 69 crossmember? Thanks.