I have the Vintage Air heater in my '31 (I did have a VA air unit in the same place prior) and while they are fairly big, if you do the installation before wiring and gauge set up, you can hide them.
http://i70.photobucket.com/albums/i8...d/DSC_0232.jpg
http://i70.photobucket.com/albums/i8...d/DSC_0235.jpg
There are two dash vents and a huge floor vent that is like a hurricane when it's on high. I did have to cut the control off and have to manually open that floor vent as with an 'A' you do run out of room to loop a cable real quick.
http://i70.photobucket.com/albums/i8...DSC_0004-3.jpg
This is the firewall fitting - a modified 4 hole a/c version:
http://i70.photobucket.com/albums/i8...DSC_0016-1.jpg
The way my car firewall is set up, I couldn't install one like Steve has in his DW's car