If the 1800E is a '70, that means the trans may well be the M410, not M41. That means the bellhousing is part of the trans, and that bellhousing requires a cable clutch. I don't know how you'd convert it to use the existing hydraulic setup.

The drive shaft is different and won't adapt to a '65 readily at all. The output flange on the OD won't mate with the original, shortened drive shaft, although a good drive line shop should be able to cope with that.

The trans support member isn't any different in 122s with or without OD -- I've never had any need to reinforce one. I would recommend using a HD trans mount, though (readily available).

So, it is really is a M410 (gearbox-to-bellhousing bolts not visible), what I would do is buy it in a heartbeat... then sell it to some racer for more than a good M41 would cost (or trade it for a M41 plus cash).