Early model HPE heater core/ac unit removal

[mwredit]

I'm attempting to remove the heater/ac unit in my '77 HPE to replace the heater core.  I've removed the 4 upper attaching nuts under the cowl, the heater hoses and ac lines.  I can only get the unit to drop and wiggle 1/2".  It seems as though there's another bolt or two holding it up, but unable to see through all the wiring, glovebox, etc.  I've consulted the factory service manual and the very vague Haynes manual to no avail.  I've scoured the firewall for extra bolts, etc and found nothing that would hold this unit in.  On most US made cars, you have to remove the whole dashboard to replace the heater core.  I hope I dont have to do this here.  What am I overlooking here?  Any suggestions?

Thanks,
Mike

[HFStuart]

It's a long while since I've done this but I've got the feeling the dash has to come out  - the good news is it's a pretty simple job.

[mwredit]

Update...The heater/ac unit is now out.  The back of the unit was stuck to the firewall insulation, which is coated with a tar-like substance and hardened with age.  I gave a little more force than I did the last time with a tug from the back, then the front and with a slight twist.  That did it.