Ive done all of the conversions out there,
What about a 8HP70 conversion? I would choose 8 speeds including OD over a 3 speed with a GV's. These where put behind the hellcats so they will just about handle anything you put in front of it.
link This is another option.
The GV gives you 6 gears. Do you really need more behind a 500+ inch stroker? That 8 speed will be big $$$ by itself and then it needs a $tand alone controller that I'm not sure even exists and if it does, is it readily available.
I would use the GV kit for the short tail motorhome trans to keep the overall trans length and driveline angle reasonable.
I think they also make a shorty kit that uses the married 4x4 output shaft to make it a really compact installation.
Kevin
