My powerwagon was exactly like that... big puff of smoke on start up. Pull the valve covers and see if the seals are even there. Some valve seals and valve cover gaskets are cheap. I'd go with poorboys suggestion with a compression test first, just to make sure the cylinders are good.
That is the first thing you should do (1) Compression test - dry (2) Compression test - wet (2 squirts of oil in each cylinder before testing) this will determine good rings or bad, if no change and still big variation in numbers then you have a valve sealing issue. Testing is cheap and will prove a lot.
