Hey I know this is an old topic but you can use the bearing retainer from a later t10 which is slightly larger than the 63-64 version and use the 6 bolt bell housing.ive actually done this and also swap the other way to run a 66 t10 with a 5bolt bell housing.
If that's a true 390 specific Toploader, it's not a big block unit. It's a small block Toploader with a shorter input shaft. None of the 390's got a big block Toploader. And if you retained the stock small block bellhousing, your input is unsupported now, the front snout will not reach into the pilot bearing (or bushing) unless you used a Boss 302 pilot bushing. If you did not do this, it's just a matter of time till the front bearing is destroyed, along with the input shaft's teeth. Small block Toploader behind a big block ? No problems there. I ran them behind a 390, 427 and a stroker 454 (427 with a 428 crank) in a 67 Stang in the 80's.
