When I swapped out my IL-6 with 3OTT for a SB and a short tail shaft Turbo 350, I didn't even know those holes existed!
I changed out the transmission cross member, getting rid of the front riveted one, and the original driveshaft length was perfect!
WHEW!
The engine sits back further, and my HEI just clears the firewall, but that helps with the weight transfer upon acceleration so I'm happy.