Larry -

No suggestions, unfortunately. I did mine on the bench - I don't even remember the two screws causing you problems. To the best of my recollection, you are the first to attempt a heater core replacement with the box in place. I do recall how tightly the box tucks up under the windshield base. Maybe we can get a professional tech to chime in.

Worst case, it's not that hard to pull the hvac box. Removing the console sides isn't much work. The radio just unplugs. You already have the radiator drained, I presume. So it would just be a matter of evacuating the a/c system and disconnecting the expansion valve. The heater box itself is secured by five easily accesible nuts; it just need be pried away from the firewall. I'd guess it's about an additional hour for you to just pull the whole box.

Of course, then you need new o-rings and probably a dryer. And the expansion valve can be uncooperative when it comes to getting a good, leak-free seal. Plus side would be the opportunity to pull the evap and give it a good cleaning.

Best of luck,

- JimY