CDAUSA, Welcome to the site. Sounds like a great car. There were 3,765 L78 396/375 hp Novas built in 1970 and 1,802 396/350 hp. I don't think that any records exist that can tell you how many 375 hp Novas had the TH400. I would guess most of them were 4sp. Is there a plate on the pass side of the trans with a bunch of numbers on it? I was trying to decode these plates to see if the serial numbers will show how many were built but I don't have enough data yet. here is a Nova site with more info
http://www.novaresource.com