It appears my heater core need replacing on a 1970 Dodge Challenger Covertible. I was given an estimate of $400.00. I use to be an Aircraft Machanic and studied a year of automotive machanics. How difficult is changing the heater core?

Lately, I have not had good luck with local Machanics. I just has a guy give me a $1200 dollar verbal estimate for replacing a head gasket and rebuild my carb, and hang a gas tank. I supplied the parts, and he gave me a final bill of $3200.00. I was shocked. I had to to get my car back, but I thinking about bring a small claims lawsuit. I don't mine paying a fair price, but $3200 and he did not even do the heads. They were are ready done. The job should have been under $1500. Sorry, now back to the heater core? Can an everyday part-time weekend machanic do it!