As far as I know there are two types of top loaders when it comes to length, short ones and long ones. Short ones come in mustangs , so if this tranny came out of a mustang you should be OK. Also , since this is out of a 390 car you tailshaft housing will be wrong for the Hurst shifter used on the '70. Take the tailshaft housing off of your other tranny and use it on the 390 trans. While you are at it you should replace the seal in the back of the tailshaft housing and also check your speedo drive gear. I am assuming you already had a 70 4-speed toploader in the car, looking further at your post you mention that this is a 4-speed conversion. So I'm not sure if you already had a '70 4-speed and '70 shifter assm. , if you didn't just ignore that tailshaft housing business. Also if you still have the old toploader they are very easy to rebuild , you might want to consider this because the trans you just got may also need work. It's not uncommon to have to replace syncros and second , third gears as well. These trannys are 30 yeaars old and have usually had their share of wear and tear.
Good luck
70 SCJ R-CODE MACH 1
88 GT