Bernard...You probably utilized the lower mounting bolts/hydraulic fittings, that came standard on the first generation(1976-79) Mercury V-6 O/B, in Figure 1, Item 15A, above. They were still available in the early eighties. Maybe you came up with the idea in the field as an answer to the first generation 3.4 gearcase water intrusion issue, that was found to be caused by a water pump base design, that leaked? Mercury updated the 3.4 water pump design two or three times in the first years of the consumer 3.4's.
http://www.mercstuff.com/images/reservoir.jpg...along with the 70's/80's MerCruiser drive reservoir bottle...?