I think it is completely "do-able" but it's gonna be a tremendous amout of work and it will cost $$$$$...probably more than you can but a genuine Mitchell preban YUGO for, on top of that the Century builds are pretty good shooters from the git-go. The Tantal idea is GTG...
Let me understand this correctly -
You want to cut up and destroy two Century M70s to use the parts to convert two Zastava PAPs?
I am guessing your M70s are slab-sided with plastic handgrips? While the Slab-receivers are not "Yugo" in sprite, they did exist as military rifles.
As mentioned, if you are want a fantastic YUGO original rifle, sell your current ones, save the +$1,000 in labor costs and just buy one.
Mitchell's imported them about +25 years ago and plenty are still out there in fantastic condition. Expect to pay $1,200 - $1,500.
So sell the two underfolder ($1,000) the PAPs ($600) and take your +$2,000 in labor and bamn you have nearly $4,000 to buy your two dream Yugo rifles.