I'm with David, you should have bought a .85 model camera in the first place.

I shoot with two M6 TTL. Once is their .58, and the other is their .85. When buying my system, I planned out my purchases with this goal in mind.

Why not add a magnifying eye piece to the .76 when you need magnification? Leica makes a nice one. Far cheaper. I use one on my .85 when shooting with my 75 mm, and 135 mm lenses.