Re: can't decide between late 70's evinrude or mercury
Aside from the fact that so much depends on the condition/compression/maintenance histories of the respective engines, I can tell you that the Johnson Evinrude 70 HP "triple"/3 cylinder is one of the better outboards that OMC made. I have a 1976 70 HP, and it is a great engine. I bought it 19 years ago, and had to rebuild it after about 6 years, but it is a bullet proof engine when maintained.
I've owned inline 6 and V-6 Mercurys, but have no experience with their 70HP offering of that vintage. I love the Mercs, but you will not go wrong with that Johnson Evinrude all things being equal. Still many of them around pushing pontoons and small runabouts.
One thing re: the 70s Mercs: There's a good change you may need to replace the engine wiring harness as the insulation has tended to deteriorate and crumble after 20-30 years as many here will attest.