Being from the middle of the Canadian prairies, I've had some experience with cold weather and frosted windows. A couple of things you can do-

A van style rear heater will make a big difference. You'll have to run long heater hoses so its a good idea to insulate the hoses to avoid heat loss.

Make sure the air being heated is fresh air! If you have a "recirculate" position on your heater ( which probably wasn't an option on your Burb ) DON'T use it in the winter. If you do you're just moving around damp air. You need to bring in fresh outside air. New vehicles now even cycle the A/C compressor to remove moisture from the air.

Get lots of air moving around. I had an old Land Cruiser with this problem. I cut the heater box out and installed the biggest heater motor and fan assembly I could find into the hole.

Sealing the body up too tight might be the wrong thing to do- sometimes rolling a window down a bit to bring in fresh air helps. Lots of houses here were built real tight a few years ago for "energy efficiency" at 40 below. Unfortunately they were so damp inside in the winter people were having mold problems. Now they use heat exchangers or plumb in lines to the furnace cold air return for fresh air- mine has a 5" line.

Hope some of this helps!

Dave
66 GMC- father /son project with the "GMC KID"
36 Chevy PU- 4x4 lifetime project
64 Skylark- wife says its hers