My '91 has the drain on the block toward the rear, passenger side.

I used to do multiple flushes, but since the coolant comes out clean, I just do it once anymore. The trace of old coolant is not worth the extra hazardous waste generation. I too use Prestone 5/150 and I mix it with distilled water. So far, no problems.

Remember, the reason that you change your coolant is because the rust inhibitors are exhausted after a couple of years (5 years for 5/150)

245 190k mi 328 BMW 60k mi old Toyota Starlett 250k mi
(nothing but RWD in my garage!)