that must be a later model 235 in the top picture, which is good because it has the side bosses for side motor mounts
you can see them right behind the generator in the top pic
the motor is set up with the front motor mount and bell housing trans mounts like what was original to your 58
when you change over to a later trans with the rear mount you need to use the side mounts on the motor
the x-member you have looks like a sbc x-member, you could hang the motor and see what modification you need to do to the xmember
or just start from scratch with flat plate similar to what parkwood did here
and here
