Another alternative idea: you probably won't be comfortable with this, but check out systemd-nspawn. There are lots of examples online, and even I wrote about how I use it: http://raw-sewage.net/articles/fedora-under-centos/
This is unfortunately another "sysadmin" solution to your problem. nspawn is the successor to chroot, if you are at all familiar with that. It's kinda-sorta like running a system-within-a-system, but much more lightweight. The "slave" systems share the running kernel with the "master" system. (I could say the "guest" and "host" systems, but those are virtual machine terms, and this is not a virtual machine.) For your particular case, the main benefit is that you can natively share filesystems, rather than use NFS to share files.
So, it's clear you have network capability between the C6 and C7 systems. And surely you must have ssh installed on both systems. Therefore, you can transfer files between C6 and C7. So here's a way you can use systemd-nspawn to get around trying to install all the extra libs you need on C7:
1. On the C7 machine, create a systemd-nspawn container. This container will "run" C6. 2. You can source everything you need from the running C6 system directly. Heck, if you have enough disk space on the C7 system, you could just replicate the whole C6 tree to a sub-directory on C7. 3. When you configure the C6 nspawn container, make sure you pass through the directory structure with these FTP'ed files. Basically you are substituting systemd-nspawn's bind/filesystem pass-through mechanism in place of NFS.
With that setup, you can "probably" run all the C6 native stuff under C7. This isn't guaranteed to work, e.g. if your C6 programs require hooks into the kernel, it could fail, because now you're running on a different kernel... but if you only use userspace libraries, you'll probably be OK. But I was actually able to get HandBrake, compiled for bleeding-edge Ubuntu, to work within a C7 nspawn container.
That probably trades one bit of complexity (NFS) for another (systemd-nspawn). But just throwing it out there if you're completely stuck.