I'm pretty sure the heater core in my GT is leaking. Orange fluid (GM Dexcool) has leaked onto the legs of at least one unfortunate passenger in my car, and when I first turn on the heater, I notice a bit of a sweet smell. So I'm pretty sure it's the heater core.

My questions: Is the heater core a job for a shade-tree mechanic like myself, or is it better left to professionals?

And, roughly how many hours does a heater core replacement in these cars take?

1/2 hour job. Remove the hoses from the core stems from under the hood. Remove the four screws holding the cover over the core from under the dash and remove and replace.

Why dexcool??

Fleshrocket,
I can't address a GT directly, but on my 84, it took about an hour. There isn't much to it at all. First, you may want to drain some of the coolant, but will probably be ok even if you do not. Remove the windshield washer reservoir to provide easy access to the 2 heater hoses. Remove both of them. Inside the car, pry off the passenger's side speaker grill on the dash. Be carefull, the dash may be a bit brittle, & crack. I put a peice of pasteboard under my screwdriver to asorb the force. Remove the speaker. 4 screws. Heater core cover is held in with 4 screws. 7mm socket will fit them I believe.
Removing the speaker will allow easy access to the top rear screw on the heater core cover. Remove the other 3 from the bottom. Once the cover is removed, There is a metal strap that holds the core in, 2 screws. Remove them. Pull the heater core out, twisting it a bit to clear the lower dash. There is a piece of foam that goes over the 2 tubes, it should be put back in with the new core.

How many hours? Oh, about 1/2. Yup, easiest heater core to replace I've ever seen. The Chilton's and Hayne's books pretty well have it covered. The only tricky part is getting all the air out of the cooling system when you refill. Hit the link at the top of this page labeled "Ogre's Cave" and go read up on Cooling System "Burping". It'll tell you everything you need to know.

The heater core itself is cake. Two hoses near the windshield washer fluid reservoir, 3 screws on the cover under the dash, and it pops right out.

Make sure you put down some towels or newspaper to catch the fluid that is going to drain unto the passenger footwell carpet. If you caps the hoses when you pull them off it will make less of a mess. When you reinstall it, connect to lower hose first, then you can fill it carefully with another piece of hose and a funnel. Once it's full reconnect the upper hose. This way you don't need to burp the cooling system.

Thanks for the advice, guys. I'm gonna get this thing swapped out ASAP.