replacing heater core

Need to replace heater core on a 1994 SE that has 160k. Can this job be done in one evening ? Is it hard to do with this style of car ?
Temp gage shows 190. Thermo has been replaced.Cooling system was flushed last winter but didn't help. You have to drive car 10 miles to get warm air out of heater.

yes you can remove ecm air bag computer and heater core housing.. its a tight fit for housing but it does come out with out removing dash.. it took me 2 1/2 - 3 hours.it was my first time in a bonn.. i posted questions on here.. its labeled.. heater core??