Cast iron should be ok Norm as long as it slides over the boss and just silver solder it to the boss or braze it , I would assume you would remove the boss for welding so heat doesn't melt the lower seal.
The standard oval blade plate is about 1300 grams without blades so an extra 300 to 500 grams on the boss should be no problem.
If you have got a bigger standard boss on a motor already it would be easier to use that ,just look for a PT with a straight intake manifold instead of the standard offset one.
