Firewall question: Conv vs. hardtop

[JonKrisInc]

I have removed the A/C components from my 72 Cutlass conv., and am trying to replace it  with a standard blower motor and case. I got a blower motor case from a 72 Cutlass, but it does not fit my car. Is there a difference in the firewall between an A/C and non A/C car?   Bruce 72 Cutlass Conv. 56 Mercedes 190SL Conv. 05 Caddy STS 04 Caddy SRX