I'm not real sure that the trans was coded in the v.i.n. in 74, but every Ford built during the 70's had a code sticker on the drivers side door jamb. If you could get the trans code from that or get a picture or two (including one of the pan) from your trans. I'm sure someone here could tell you with certainty what you have.
I have a 71 E300 14' box van with a 302 and a C4 automatic. So they did use the C4 in some bigger trucks.
I think all the C4's unbolt from the bell housing, but the C6 does not. There casing is molded to bolt to certain size engines.