No you can install a plain heater box in place of the A/C box. I did this swap in my 71 Scamp. There is however a issue, the A/C blower motor is larger than just the heater blower motor. You will need the trim ring for the plain heater box. It is round and it is almost as large as the opening for the A/C motor opening.
I also had to make a plate to mount on the inside of the car between the heater box and the fire wall. That went over the studs of the blower motor. This was to fill the large opening and give the box some support, and something to pinch the box when you tightened it down. I then pulled up as far as I could on the heater box and tightened the trim ring on the engine side of the fire wall. This held it in place very well, It has not moved at all.
As for the holes where the A/C lines when thru and the holes from the bolts for the A/C box. I ended up getting some body plugs that fit those holes and pushed them in. Worked very well.
Of course to do this properly you would get a section of firewall from a donor car that only had a heater box and weld that in place of the opening for the A/C. I did the same thing as you and decided after I painted my car that I wanted to get rid of the A/C.
I guess this way if I ever wanted to add it back in all the holes are there.
