Larry makes a good point. There are some rather pricey devices that in effect, in seconds, measure toe-in (Side slip): http://www.hunter.com/pub/product/product.htm
see inspection lane equiptment - Side Slip
I would not consider this to be a front end alignment as they don't read caster or camber. On the car in question I would only do a "front-end " alignment also. It has a solid rear axle that can not be adjusted (I would measure it only if looking for a problem or after an accident).
Four wheel alignment is in most cases used even if only the front can be aligned. The purpose is to establish a thrust angle (the direction the rear wheels are pointed. From this the centering of the steering wheel can be attempted during front end alignment. This is the only reason to install the rear heads.
This method of centering the steering wheel is too inaccurate for my tastes and I have devised my own technique so I often only do front end alignments on cars that have no rear adjustment for less money. It takes me about 15 minutes to align every factor in the adjustable MB front ends. It adds about another ten minutes to do measure/adj the rear. With test drives before and after the actual difference in time is about 25% of total. I charge accordingly.