Yes, your bell housing should be vertical where the motor mounts bolt on. I believe the crossmembers are the same over the years, although may have changed a bit when the flat top beds changed the rear hump height. The crossmember where they bolt on should be angled (not horizontal). Two bolts go into the bell housing and one through the crossmember.
My motor mounts ('51 3100) should be the same as yours, but they're not accessible right not or I could post some measurements. I don't think it matters whether the sides of the mount are sloped or not, as long as the bolt holes line up.
