FWIW I've always been more than a bit leery about that era of simplex long-arm mech; I've seen plenty go into the rear wheel or just break for no good reason (*). However for many years there wasn't a good alternative, not off the peg anyway; eventually campag started to make a long arm 'Rally' mech but until then folk were a bit stuck if they wanted a robust RD that mounted to a 'modern' gear hanger. So for a long time a common modification was to take a RD with a good parallelogram design (i.e. not made from worryingly floppy plastic) and to extend the arm somehow. The campag Nuovo Record mech was pick of the bunch for this, and (just like the later production Rally models) the guide pulley was usually moved downwards/backwards to allow a larger sprocket to be used.
I have seen a few mechs like this and some are beautifully made and others pretty much look cobbled together. The one below (picture found on the interweb) is a bit of both and (allegedly) came fitted to a Rene Herse.
It uses some simplex parts and some DIY parts in the cage, nailed onto a campag parallelogram.
(*) It is quite telling that in the UK especially, machines which used the widest range gears, such as touring tandems, were still commonly fitted with the somewhat archaic chainstay-mounted 'cyclo standard' RD (or similar) right until the late 1960s or early 1970s; these were one of the few mech designs that had enough capacity and were likely to last more than five minutes.
Anyway if you want a 'period correct' bike with wide range gears then both conversions of mechs and chainstay mounted RDs are fair game.
cheers