21.75, 24.5, so just under 3". problem is bolt circle pattern and output shaft spline. a 350 is 27. a 400 is 32. a 400 has a small circle 6, a 350 has a 4 bolt. what tcase and adapter are you running? thing is you can have an adapter made to put your case on a 400 and keep the same drivelines if you have a plate made and get a 400 off a 76-79 chevy 3/4 ton that had a 203 behind it. this is the shortest output shaft of a 400, and the plate only needs to be 5/8" thick. if your adapter to your atlas (I'm assuming here) is like 3" long, you could be in business. you countersink the holes on the tranny side and the tcase bolt heads are exposed. just did my 350/231 this way out of 3/4" plate. works like a charm, 1 hour on the lathe and some drill press time. yes, you will have to change mounts, but that is the easy part of this. the adapter plate and input gear swap out are the hardest part. same torque converter fits, same bellhousing, same shifter and same tcase shifters if you go with the short ass shaft and plate.