Wow, its a good thing I checked the blower motor to firewall fitment! Many months ago, I bought a new blower motor for the heater system. I recently read somewhere that the replacement motors are larger than the original motors. So tonight, I looked through my parts storage room, and found the new motor. I put it up to the hole in the firewall, and sure enough, its way too big. So, I grabbed the tin snips and increased the hole diameter by about a half inch. The adjacent smaller hole for the electrical connector had to be trimmed larger as well.
I'm just glad I discovered this BEFORE the firewall was painted! I'll make a sealing gasket out of closed cell foam.
BTW, the tub is scheduled to be painted on June 14th.