There is probably a kit you can get for a tri5 to do that.I have used angle and square tube ,its very simple.I used 2 pcs of angle about 3inx3in,cut 3or4in long. These will be welded to inside of frame rails. Use 2x2 square or heavy angle,set everything up under the car,then remove and drill holes. get back under and mock up .weld angles on, bolt together. I tried a junkyard crossmember,but it needed too many mods. good luck
A 2nd Gen '70-'81 Camaro makes a great crossmember to use for this. There are aftermarket crossmembers for a Tri 5, but they're a lot more money. Usually get a 2nd gen for $10-$15, then just use some angle iron to make a bracket to bolt it to.
If you can't find a used 2nd gen, here's 70 hits on Ebay for aftermarkets from as low as $29. http://motors.shop.ebay.com/__?_fro...d=1955+chevy+crossmember&adgroup_id=645768291