A lot of guys seem to use the third seats out of suburbans. Second row seats from 80's-90's dodge caravans work also, they work in Model A's also...I put one in a '30 sedan it worked great.
Seats are dimensionally the same '35-'47. I modified the back seat from an '86 Ford Tempo for by '38.
Dodge minivan bench seats look like they belong there and are comfortable, I've had one in my 36 for sixteen years.
thanks guys a friend of mine suggested dodge caravan also i thought that would be to easy there are millions of those in junk yards.