I have a 1966 Buick Chassis Repair Manual, which has every spec imaginable for your engine, but won't be back home until tomorrow. If nobody else has replied, I'll post your info then.
Here Start with bolt just forward of the carb on drivers side. Cross straight over to the pass side. Back over and rearward then cross over again. Then move to bolt just forward of the first one and then cross straight over. Then to the drivers side just forward of the water passage and cross over. Repeat pattern on remaining 4. Clyde.
The Chassis Repair Manuak specifies to install the "open" center bolt holes first. There is no other sequence. The longer bolts belong at the forward location. Torque bolts alternately and evenly to 45-55 lb. ft.