Installed a new OEM vacuum advance distributor on my Feb. '76 40.
19100-61180 OEM work of art:
My stock vacuum retard distributor looking 44 years old. I've only changed the points, cap and rotor in 18 years of ownership.
I put the transmission in 4th gear and rolled my rig until the BB on the flywheel was at the pointer and the rotor was pointed between #3 and #4 spark plugs, about 12:00 perpendicular to the engine block. I Forgot to take a picture of this (DUH!)
Removed my stock distributor and noted the rotor spun clockwise from 12:00 to 2:00 because of the camshaft gear.
Lined up the new distributor and rotor (@ 2:00) It went in the block without a problem and the rotor spun counterclockwise back to 12:00.
This is as far as the new distributor went into the block:
I've read the horror stories of a distributor not being seated far enough to turn the oil pump, so I removed the new dizzy and noted the position of the oil pump slot in the block. Re-installed the dizzy and rolled the rig (still in 4th gear) so the rotor moved 1/4 turn. Removed dizzy and noted the oil pump slot also moved 1/4 turn.
I didn't note how far my old distributor was seated in the block before I removed it (DUH! again), but there is a mark from the clamp on the shaft that suggests it wasn't fully seated either.

Re-installed the new dizzy (getting really good at adjusting the rotor for camshaft gear compensation) and connected spark plug and coil wires.
Fired it up and made sure it had oil pressure (all good), then set the timing to about 10 degrees BTDC.
Checked dwell first, because I forgot to check the new distributor point gap before install (3rd DUH!)
Got 43.6 and FSM spec is 39-43 for my rig so I'm not going to mess with it.
Next up, vacuum and idle rpm. Steady 19.5 Hg @670 rpm. I'm stoked.
Used a vacuum "Y" from
@ToyotaMatt (Thanks!) to connect vacuum advance port on the new dizzy with my carb port and my smog VSV.
Yes, my rig still has a full smog system

.
I haven't road tested it yet to see how having vacuum advance instead of retard is going to change engine performance. I had my timing set at about 12 degrees BTDC with my old retard dizzy, so I'll have to see how it drives now before I adjust it.
Any suggestions/ideas on setting timing or the new distributor not seating fully are appreciated.