I would vote for a Mavic Module E with the Mavic red only label. According to Velobase, this was in production 1975 onwards, but certainly I have seen these on late seventies/early Eighties Peugeots. A polished NOS Module E clincher rim - yes please! Time for you to get the Ebay googles on
On my '85 Merckx I was looking for Mavic MA2's, but these are pricey, but ended up with some polished Mavic 190FB's on DA 7400 hubs - very nice too.
As for sizing, again my retro Guru is the best place to start:
And for a general article on tyres:
For anyone that hasn't come across Sheldon, he passed away in 2008 (check Wikipedia out) but you can spend a lot of time exploring his website - its packed with Retro info.