I would stick to a stock crossmember... keeps it easier. With the sidemount 270 (1956-1959) Do you have the horseshoe mount? If you do, you may be able to drill a hole in the chevy crossmember for it, or bolt a plate to the crossmember for the horseshoe mount to go through.
If it were me, I would change the front xmember to the gmc one, and change the front timing plate so it would simply bolt in like stock. The front timing plate from the 216 will interchange with the 270.
