A "Mopar Man"that cannot distinguish between a big block and an LA series engine shouldn't be touching your car. NO Chrysler 400 or any big block has the dizzy in back, I don't care what numbers* your "mechanic" sees. Plus what wagon killer says, a 400 would have a 727 behind it, not a 904. Yours; https://en.wikipedia.org/wiki/Chrysler_LA_engine "big block":https://en.wikipedia.org/wiki/Chrysler_B_engine Casting # for your engine is on the left side a bit below the block's deck. They are raised #s, the casting # followed by displacement. Ask him to show you yours.
I was going to recommend having a Transgo shift kit installed before he reinstalled the transmission, but I don't know if I'd let that guy change my oil.