Either the Steele or local rubber will work. The important part is to have the single hard contact at the middle bolt on each side of the cradle and have the other bolt locations cushioned so if there is some flexing and twisting the movement will not break the seams. The steel washer at the center hole both spaces the flange away from the cradle and provides the solid connection while a steel sleeve or spacer is used on the bolts at the rubber cushioned holes so the bolt cannot be tightened so much as to compress the rubber too tightly and make those the same solid connection as the center bolt.
The cradles were identical 51-54. After the modification was made to the early 51 radiators per the bulletin (cutting off the top of the flange and adding the various cushion washers and spacers) the mounting method and parts as described in the bulletin stayed the same thru 54. 55-6 had a different cradle and radiator width but the mounting also stayed essentially the same as the earlier cars.
I hope all is well. I have scoured the forum in search for a straight answer on the optimal oil weight for my 1954 359 L8. To no avail. I currently use AMSOIL 10W-30 synthetic ZROD oil with a FRAM C134PL by-pass oil filter. I have also used a WIX 51080 filter. All works well. AMSOIL ZROD oil comes in 10W-30, 10W-40 and 20W-50. The car is only driven from spring to fall. The average speed +/- 40mph with some pinging north of 55mph. What are the pros/cons of using 10W-40 or 20W-50? Wouldn't 20w-50 provide optimal protection for such and old engine and high heat? Would the old oil pump be stressed with 20w-50?