There is a pic of dimensions of a block that was made but was weak as I remember. Made By JB maybe for his loop/EV's.
You got the right idea though. Loop & Tonti swing arms are exact same width at frame pivot pins.
I believe you're thinking about Mike Tiberio's first version of adapters. https://www.thisoldtractor.com/moto_guzzi_loopframe_brackets_to_mount_a_tonti_timing_cover_in_a_loop_frame.html
Those cracked, so Mike made them in two pieces, which solved it. I have a pair I don't need if anyone is interested.
One can use the crankshaft mounted alternator, but the fender brace will need to be rotated up or down for clearance, and possible the crown of the fender flattened as well (like Guzzi did on the Cal 1100 fenders). If you want a generator or alternator in the "V", swapping on a Loop timing cover is possible (timing chain Eldo will bolt right on, earlier will require removal of internal webbing). That eliminates the front engine mount issue, but then you'll need to figure a pulley hub/crank adapter. Plus the crank will likely poke out through the Loop belt cover unless it was shortened somehow.
Was there ever a Cal 1100 engine that came with the Bosch or Saprisa alternators? The 1100s are a little past my focus. If there is an 1100 with the short, tapered crank nose, then there is a ready made adapter. The top of the crankcase where the generator/alternator bracket mounts may require machining flat (if it's as-cast), holes drilled and tapped for the mounting studs.