1966 Cadillac Hearse Driving problems

Engine Performance problem
1966 Cadillac Hearse V8 Automatic 800000 miles

while driving the engine seems to stall. For example while driving on the highway im going steady 70 then stalls and drops to 50 (this is all happening my im holding the gas pedal down) then it will pick up again in a minute or so. It has white smoke coming from the exhaust and all. And it starts to smell bad when im driving long distances.