VW was very smart when they fitted the 1600 Single Port to the early VW Busses. The single ports are less prone to cracking between the valves with the added weight and heat from a heavy bus. Correct me if I'm wrong but the Twin port 1600 was introduced to combat US smog laws which sucked the life out of an engine.
My favorite Bus engine would have to be a wierd combination of parts. I'd use a 74 stroker crank and stock 85.5mm Pistons and barrels, with single port heads and Dual Kadron carbs and a quite pack exhaust. That way you get a bit more torque ( Hp won't move a bus much faster, they weigh a ton and have the aerodynamics of a brick) with stock heads, and if you are lucky no case machining if you get a good crank. The dual carbs will liven it up with a free flowing exhaust. Now I'm sure this engine won't apeal to everyone but it should last quite a few miles and return favorable petrol mileage. Oh and a SVAC distributor no 009 for me.