Welcome to CACC. It is great to meet another do it yourselfer. That amount of time does not sound unreasonable. The reason is, the heater core is inside the car. You have to disconnect the lines on the engine side, then snake up under the dash and yank the main unit down to get to it. In fact, this may take a first timer an entire Saturday to get it in and out as well as purging the air from the coolant and getting back to normal.
I would plan on a long weekend, new heater hoses and clamps, plenty of fresh coolant, and a good manual to help guide you in and out faster.
Chris is absolutely right. And..... if you're not real careful, you'll end up with rad fluid all over your carpetting. Take some extra care and put some plastic over the carpet BEFORE you pull it out. Lesson learned the hard way on my '72. I thought I'd blown it all out first. Not!
I almost feel like an expert here. I've done this job twice! Learned on my friend's 76 and then did my own 77 during my engine compartment resto. Its really not that bad. 1) Plan on a full day. 2) Get your core from The Last Detail or GM. The one I ordered from one of the other big name vendors did not fit into the cradle properly (its critical that the in/out pipes are bent just right). 3) Remove the passenger seat. Makes it a whole lot easier working in there. 4) Plan on cutting the heater hoses off the core pipes with a sharp utility knife (score it straight down from the top, after you've dropped the clamps of course). Don't wrestle with these hoses. Its not worth it. Get new hoses. 5) When you cut the hoses, keep your face away from dripping coolant! Also, place a drain pan under there and quickly bend the hoses down to drain the remaining coolant (there will be a lot). 6) Put the front of the car (or just the right side) up on stands to remove the hoses. You can drop it back down after they're off. 7) Put some old towels down on your carpet like stated above because when you tilt the distributor box out of the car, it will drip a little. 8) Remove and clean all the ductwork. Use a big bottle brush to scrub the insides.
As for the actual procedure... just wing it! No... I THINK it goes something like this. This is for an A/C car. Much easier for cars without A/C. You can actually get at it from the engine side!
1) Disconnect battery. 2) Drain radiator. 3) Remove hoses from core. 4) Remove nut from stud that sticks through firewall (upper right as you look at it from the front). 5) Remove psngr dash pad. 6) Remove center console and radio. 7) Remove 3 large screws holding distributor box to firewall (one in each corner). 8) Remove a couple other screws holding the ductwork together. All very reachable. 9) remove center duct and floor outlet duct. 10) Detach temp control cable. 11) Tilt distributor box down and out. Remove vacuum hoses as you go. 12) Dis-assemble box and remove core. 13) Re-assemble in reverse.
When you put the box back in, be careful not to pinch or bend over any of those little vacuum hoses. If you do, your flapper doors won't open and close properly. Re-adjust the temp control cable.
I also take notes whenever I do a major project. Much easier to put things back together, especially if you have to wait a day or two. Old minds tend to forget rapidly.
I have been running for 3 years with NO HEATER just to avoid replacing the heater core in my 80. I don't drive it in the cold anyway, so not much of a problem. But, I really need to do it and this winter might be the time. Currently, the heater core is bypassed.
Best reason to do it yourshelf is that you know everything went back in better and cleaner than it came out.
Never done it, but here is my .02 worth. I have the firewall chopped out of a 79 here at my shop with a/c. The a/c hole is larger than the heater only hole in the firewall. So, you will have to make a plate, or fiberglass a patch in the different. It is not a big difference, but just enough that the stock heat only unit will not cover it all the way around. It would really be easier to go from heat only to a/c as it is always better to cut a hole out than fill one in.
Other than that, as long as the control head gets vacuum to the right places you should be off and running. There are not that many other differences that I am aware of.
Hey, if you are into that kind of work, why not join the few and the brave of us that have installed Vintage Air systems? When done right, it fits under the dash and leaves virtually nothing on the firewall. You are able to retain both heat and air in a more efficient compact unit than the stock system could ever imagine producing.
If you do the work yourself, have about $1,250 to $1,500 set aside. That will cover the under dash unit, hoses, drier, lines, condesor, brackets, compressor, etc. etc. to complete the system including a cool new control head. The Vintage Air unit is all electric so no need to run any vacuum hoses.
Please note that they do not sell a direct fit unit for a C3. It is a custom install using the Gen II Super Cooler technology in the Compact Unit.
Correct me if I am wrong here, but I think you can go to J.C. Whitney and buy an electric heater (used in jeeps and RVs) that would be compact and save even more space if you are going for the race look with a defroster.
check out my photopoint link below, back into my albums and select the Vintage Air conversion to see some pics of the install.
I am after the same thing you are, but don't want to sacrafice hearing my wife complain about the heat.