Big Dan's fix will avoid fixing the heater core, and will essentially return the coolant back to the engine. If you don't mind doing without heat for the rest of the winter, do that.
If on the other hand you need/want heat, then you must address the problem and fix it right.
From what you are saying, coolant coming out from INSIDE the heater box, it points at the heater core having developed a leak. Since the hoses hook up to the core OUTSIDE the heater box, and the valve is also outside, those items being the ones that failed would have demonstrated the leak outside the box.
Short of removing the heater box and getting to the heater core, I can think of no "right" way of fixing it. I've heard of radiator "stop-leaks" that might do the job, but at best they are stop-gap remedies and work by plugging the hole from inside the core with either metal shavings, solder, or tar / plastic / goop.
Unfortunately, to get the heater box completely out, you're going to have to remove the dash. Maybe somebody else has figured out how to get it out without doing that, but I don't know of it.
You can get heater cores from the dealer, Motorsports and Victoria British, (don't know haven't checked). You might also give Z-Barn and Z-Parts a check see. Last case, check your local boneyard, or ask others on here if they would check the boneyard for you. A lot of people will argue against the boneyard, but many of the cars in there are there because of accidents that rendered the car unsalvageable through regular repair methods. The rest of the car may be in pristine condition. The last windshield I got for a Z came from a boneyard. Cost? $30.00 (in the late 80's) compared to over $200 then.
For what it's worth.