heater blows cold air

[Libby]

This is my first post, so be gentle with me. My baby is a 67 coupe that I've owned (and driven almost daily) for 28 years. She's never been warm-blooded, but several months ago the heater started blowing nothing but cold air. My mechanic seems to think the problem is in the plenum, but isn't too crazy about the idea of crawling under my dash to work on it. I had him replace the thermostat, which made no difference. The heater core is 2 or 3 years old. I had the fellow at the radiator shop look her over, and he said the upper rad. hose was hot, and the lower "sort of hot", which he said was ok, and he didn't think the system needed flushing.
There's a lot of air coming thru the vents, it just isn't even remotely warm. It isn't cold enough to be outside air, so obviously it's coming thru the system. The temp. gauge still hits the same spot on the dial, so I would assume the engine is heating up the way it should. No coolant leaks anywhere, and the level is right. The heater box & plenum are original, & the box is in rough shape. Would a bad plenum allow only cold air to come thru? My thoughts are maybe the heater core is plugged or faulty. Maybe there's a kink in a hose somewhere, but where do I look? I hate to go to the expense & trouble of replacing the box & plenum if I don't have to. Any suggestions on what I should try next? I live in the midwest, and believe me, it's cold in that car!
I don't know that it matters, but the last work I had done before this problem started was a basic oil & filter change, and new shocks installed.