Any tips for installing new heater core?

[Mopar to ya]

Last year I got a little fog on my windows even on hot days. I didn't smell coolant. Well, once in a while I could get a light smell, but I never found any other evidence of leaking. Today I put on my new heater hoses and while the cooling system was bleeding and I was waiting for the thermostat to open, I saw steam coming from inside the car. I ran my hand along the bottom of the heater box and it was wet with coolant. I know what that means, and it doesn't look fun. Any tips to make it easier? 1973 Dart Sport originally with A/C. Any help would be appreciated.

 

[1970Duster]

remove front seats (both) and console/shifter so you can lay down while having easier access.

 

[RedFish]

Factory service manual. One the aftermarket manuals might be sufficient for this job.