How do you replace a heater core on a 1991 Chevy blazer Also does the fact that it has a 1995 4 point 3 vortec engine affect the placement of the core?

1.first thing to do is disconect the negative cable from the battery 2. drain all the coolant from your radiator 3.go inside the truck and in the passenger compartment look right underneat and you will see a black air duct, take it apart from take the face out and leave the back wall intact this is the side that mounts to the firewall 4 when you get to see the heather core go ahead and disconect the heater hose that comes right of it at the heater hose, put a rag down just in case any antifrezee leaks out of the hose or the heater itself am sure you don't want to mess up your carpet ,and if your new heater core comes with plastic caps to protect your new lines try to use those to plug the old core tubes to prevent it from dreeping more. 5. Once you gain access to it the four screws that are holding the heather core, and then remove the core by sliding it down towards you. The installation will be the same way just the revese steps 5.4.3.2.1. Also keep in mind that there are electrical wires running between the dash in both ways , so make sure that no wires get pinch, or cut , double check and make sure,wires pinch or cut like could cause you to loose your ( power locks, power windows, and or power mirrors controls) so is better to be safe than sorry . Don't forget to refill your radiator with antifreze after you are done with the heater core installation and conect your negative cable back to your battery. I hope this helps you, try to start early in the morning and with lots of sun light and patience you'll be done by mid-afternoon ! good luck!