Did your 69'Merc have the venerable 390 cu.in.V8?
No, my '69 Merc (Marquis, FWIW) had a 429 cid. 7L engine with dual 4bbl Holley carbs. Got me from Tampa to Tallahassee on one tank (barely), and with a 22 gal. fuel tank, it was a road tank, to be sure.