Help with March big block system

Check to see if the pulley is not only inline (front to back) but also square to the motor (The belt will try to run to the high side) If its turned out slightly the belt will try to run to the "high" side and do what you show.
Just something to look at.
M
If the belt that comes from the crank is in good alignment then start there. Sounds line the crank to water pump also has it's own tensioner that is not connected to the main truss.
The rest of the accessories must be out.
Maybe review the install directions... Just seems the main truss is too far forward therefore the belt to the water pump that runs everything else is being pulled forward. Again , only what I can see in the pics.
I have a vintage air front runner and it only uses one belt so I have no experience with your setup.







