Depending on what you think would be easier---
You could re-drill the ford 203 and swamp in a chevy 203 input so it will slide on into you th400 out put.
Or you could find a chev 203 off of a th400 , use the case, and swap all the guts form that ford 203 in to the the chev case since it is rebuilt. Then there will be no redrilling, everthing will be nice and new and work like you want.
Another thought, since you have the newer style th400 with the 6 bolt round adapter pattern, I think (not for sure) that the 6 bolt ford and 6 bolt chevy are the same. Since ford always used a 6 bolt pattern on therr 203's and 205's, the 203 will bolt to your 400, all you need is an input. I'm just ranting here, but its a thought to check into.