If I remove the AC piping at these couplers in the firewall, is there some sort of threaded plug I can screw in to block them off? Would sticking a rubber plug in be sufficient? I’ve seen block off plates for this area but don’t want to remove those couplers because I may put AC back at some point.

 

I was in this this exact scenario. Since you wouldnt need to be holding refrigerant in, do like I did, and just get some appropriately sized silicone caps and spring clamp them on. Sorry I cannot remember what sizes are needed.

 

OP - get suitably sized rubber or silicone caps that fit snug over each male insert O-ring fitting (MIO) on the evaporator core. Since you may restore the A/C some day, the caps only need to keep air, dirt & moisture out of the evaporator core. Don't bother with any spring clamps to secure the caps though - you don't want to risk any damage to the soft aluminum male threads on the MIO fittings.