Hey forum, I am looking for the option if possible to replace the heater core for a smaller functioning unit, possibly off of a different cadillac or aftermarket unit? Reason is I need more room for bigger engine ( 5.9 Cummins turbo diesel) . I still want heat and A/c so I dont want to totally get rid of it. Also what lies directly behind the firewall? Is it safe to push it back a couple of inches for clearance, Need to push engine as far back as possible for weight distribution.