1985 Chevy Van heater core replacement

Can anyone help me with the replacement of a heater core in a 1985 Chevy Van, 305/5.0, a/c? I have the dash out and the defrost ducts and duct over the top of the dog house out, but I can't see how to get the core's box open. Is ther anywhere a person could find really, detailed info, photos, images of the process? Merry Christmas all. - MikeyG

I got it! I worked at it about 2 days straight, man, what a job. I tried to be as patient as possible, I sure as heck do not want to have to go in there again. Having said that, I almost considered changing the 18 year old Windshield Wiper Motor as long as I was in there, and saw that this is where it is located. I finally, figured out and saw that there were 3 nuts on the outside of the firewall that reomoves the heater core box. The dash was completely removed and the vents were removed and cleaned, as well as the area under/behind the dash. Prety, filthy after 18 years! All that is left is to replace the heater and radiator hoses and the thermostat and then flush and fill; still, a bit of work left!