Porky Pig has the real deal heat exchangers but calling their output anemic is being generous. The flappers close completely so that is not the issue. On my SC with backdated heat (no blower motor) and SSIs it will cook me to death. Never been able to stand it even with the levers at 3/4s.

So what do I need to recheck on Porky?

Rick the stock heat exchangers on a Six put out serious heat - similar to what you have described on your backdated car. Are the hoses all in good repair? Are you sure the flapper valves are working properly?

As Cairo said - Stock heat should be hot. Stick a vacuum cleaner hose up the longs. Mice nests can clog up the tube.

its not the heat exchangers (as others said) its the fan.

Get a more modern fan.

Grainger has 12v fans that blow way harder.

LOL.

The stock 6 heat exchangers on my car work great! Shouldn't need a fan. Something else must be going on...

The heater fan on a stock 6 is also the engine cooling fan.
At low engine speeds the output is not that great.
With stock 6 HEs it should be enough to be warm at low speeds and very warm at higher speeds.
If the boxes are patched and loose on the exhaust headers you could be loosing a lot of flow.

I am sorry, I did not know that...

yet another reason to have a 3.2 conversion car over a stock six....

LOL.

I have to disagree. Both of my '70 Sixes had killer heat/defrost functionality. I can remember driving around with the top off in winter with the heat on and being hot.

I think you may have an issue in your particular system. I would suspect rodents may have nested as pointed out above by Mark.

I can't speak to the heat output in my '71 Six as it is in pieces and when assembled will have a 3.2 with stock Six heat exchangers. But I am willing to bet it is going to have great heat too.

First off make sure your flapper boxes are hooked up and working properly. I've seen more than my fair share of boxes on cars, and the cables dangling in the air not hooked up. They rusyt and break, things get stuck and P.O.'s simply unhook the cables and turn the flappers open or closed etc. If that's all fine then clean out the exchangers inside, and all of the duct work in the car. Check the exchagers top and bottom for rusted out sections. You could also have bad tubes in the longs either form rot, or previous repairs that didn't put the hose back in. Several factors at play

Bummer, you've received good advice.
I was going to say they do suck and you need to sell them to me cheap so I can mess with them and they'll be 'good enough' on my car.

Heck even the USA heat exchangers on my 356 can pump out some heat when warmed up. The Euro spec ones (more like a 911 or 914) put out even more.

Good luck troubleshooting the above suggestions, you'll enjoy the heat when they're properly sorted.

FWIw, this all looked like a couple pinholes prior to putting it in the blast cabinet, so if you have some gunk built up clean it off

The dash controls MUST be in the correct orientation. Specifically, the cold/warm mixing aspect. Ensure the valves fully open.