85 Heater Core=Cut Box To Replace?

[5spd85GT]

I have been all over the Internet and have seen no mention on simply cutting through the box surrounding the heater core to replace it, rather than remove the dash. I had the job done on my '85 GT many years ago (early 1996) by a guy who apparently did this method sort of half-way, i.e. there is a piece (approx. the width of the core and about 1.5 inches high) cut from the front of the box and apparently he loosened things up just enough to pry up the top cover (broke it actually on the back side and that was sealed with RTV along the seam) and slide the new one in, and then cover the hole with RTV. Well, this has had a slow leak for years (anti-freeze smell in car, film on windows, mysterious slowly disappearing fluid from radiator, and most recently fog out the vents) until this weekend when the anti-freeze is literally poring out the AC pee hole right onto the exhaust (yeah, not hard to find that leak)! I was able to remove the wonderfully located 5/16 screw at the top of the core cover and remove it (as I said, the guy had pried it up and broken it for removal years ago), so the question is, why not just cut down the front of the box the rest of the way to allow access to the old core and slide the new one in? Afterwards, a little J.B. weld and some more RTV to seal things up. It seems entirely too simple (versus taking out the entire dash), so who can give a little feedback to this idea?