@mazda616 You'll be fine. Jolene is the best Sunbird around and the odds of a mechanical issue that would cancel the trip are very low given how well she's been maintained. Cars NEED to be driven to be reliable.
I usually plan trips based on "miles since last major failure." If the car left me stranded 40 miles ago, then a 50-mile trip is out of the question. At this point, I'm good to drive it to California and back without worrying about it.
If Jolene hasn't left you high and dry somewhere, fill'er up and go!
While past history does not indicate future performance, it's a pretty good indicator. By now you've got a feel for how long she can run. If it's been more than 140 miles since your last breakdown, either you're due for one now, or you'll be fine. I think the exercise will do the old girl some good.
Some of my really good friends had a repair shop for 20+ years. They've since moved on to greener pastures, but I used to recommend them to people I knew to not be difficult.
A guy I worked with was driving a 10 year old Chevy Venture that he'd bought new. He was complaining about the poor heat one winter, so I suggested he take it to my friends shop. He dropped it off one morning, and I picked him up and took him to work.
An hour later I received a text with a photo of the cabin air filter, that he's never changed. The heat was much better after it was replaced.
@mazda616 I'm going out on a limb, but I have an odd guess. I assume that there is a rod going from the temperature lever on the dash to a valve that diverts coolant through the heater core when heat is selected. Is this the case on the Sunbird? If so, could you have accidentally knocked the end of that rod off of the control valve? Does the Sunbird even have a control valve for the heater core?