I haven't done an a/c box in a while, years ago I had a chance to buy 3 NOS evaporators and I only have 1 left. (I think they fit 66 up to 70 b bodies).
But here is how I see it, if you are in there and can afford to do it right, then you must, because it sucks to do it again..
https://blog.drivenrestorations.com/2015/11/1968-1970-b-body-mopar-heater-box-with.html
I have done countless heat only boxes, and have one on the bench right now.
Heres the minimum I would do.
Evap coil/core- $250 new
Heater core- $200
dmt big kit -$120
bushing and hardware kit $25
paint-$20
Blower motor rebuild -$35
I blast and paint all linkages replace all the small parts, rebuild the motor, replace the cores and plumbing, rebuild replace all vac and valve parts, and a/c box done right normally costs me 600-850 to do myself. Heat only costs me half of that, I guess I'm lucky I don't see many a/c cars this side of the country, lol..
good luck, heater box, steering column, and dash board rebuilding is one of them skills you use once and never think about again, but need to become a professional to get it done the first time, lol...