Cusser
+1y
For sure I'd drain the coolant, and check that out. You do not appear to have a thermostat issue. Was orange coolant used, or is that color rust? If rust, I'd flush the system with chemical cleaner, then flush ALL that out with water, then add new coolant, and bleed out all the air. Yes, there should be some coolant in the overflow reservoir.
Ya' sure that it really has a B2600 engine? That's not a direct swap-over, like the transmission would also need to be swapped over, from what I understand.
Post pictures of the engine compartment..