There is a write-up going the other way on this site:
gmcguy.org
The write up is for folks with 3 speeds that want to convert to 4 speeds. I would think the procedure for your conversion would be similar. There are 2 pieces that need to be changed to get to a small hump rather than just one. The section welded at the front of th removable pan is also different.
I hope this is helpful.
Regards
Steve
New Orleans