Problems with my C4, but first my ignition...

Hi, all members ! ! !
I'm a new member, and I found your address with the help of GEETEE(Thanks to him !).
I sent my C4 transmission to a pro because I had no third on it anymore.
When I reinstalled it on my 66 289HP, I found exactly the same problem...
The vacuum modulator has been changed, before I sent the trans.
I've heard that it could be because I have an ignition problem, that brings me wrong vacuum to the trans.
I have no vacuum hose who comes on my distributor.
I installed a pertronix ignitor, with the right ignition coil.
I bought a vacuum jauge to control the vacuum on my intake manifold.
So, I'd like to know if it's right that the reading on my electronic timing light for my ignition should be 12° at iddling(700 or 800rpm ???)
Well I try to adjust my ignition, hoping it will resolve my trans problem...
Ignition adjustement, it's not really easy.....for the first time...
What do you mean about it?
Thanks for your help !
Laurent.

Yes, it's a HiPo,...sorry.
Laurent.

Could somebody help me???
Thanks, Laurent.

Hi, PB
thanks for your answear,
I've a vacuum line from my intake manifold to my C4 trans, but has I wrote, no third gear anymore on my Mustang.....
I installed a new Pertronix ignition (ignitor+ coil) that I've to adjust (...try to adjust....) but do you know if it's right that the bad adjustement of my ignition could be the cause of my C4 problem?.......since I bought my Mustang, I try to resolve this problem... .
Laurent.