What do i need to convert to single rail 4 speed from 3 speed column manual behind a 250 cross flow, also can i use the same box and just change the bellhousing when i drop a clevo in?
Thanks

Should be able to use the same box. My old XC had a 4 speed single rail in it and we fitted the VJ Valiant (was 3 speed auto before) with one out of an XD Falcon 6 (I think).
You'll need the yoke for the tailshaft (I dunno if maybe you'll need the entire shaft as boxs might be different lengths). You'll also need to cut a hole in your floor probably.
I don't know anything about the toploaders, never driven a car with one, but arn't they a better box as they were fitted to the higher spec models wen't they.