I followed the directions in the service manual, which the Grimmspeed DIY does a decent job of paraphrasing:
https://www.ft86club.com/forums/showthread.php?t=9883
You have a no-spill coolant funnel? That will raise the fluid line and allow you to fill and overflow the heater core bleed.
And try running the car up to full operating temperature with heater running on max hot min blower, then allowing the car to cool down completely before you check levels again