Don't know about your Mazda, but I used to fit one into a 76 Toyota Carolla with 2 doors. Just folded the front seat down and wedged it in there.
Regular 810e vs Heritage: Heritage wins. The regular 810e is great, but the Heritage sounds like the old flatbacks with the awesome CTS alnicos and handles much more power, so for me it's the best cab Ampeg's ever made.
Ampeg Portaflex Club #1