How much would this cast iron radiator weigh?

Couple of hundred pounds, easy.

How you go about moving it depends on how far you plan to go with it -- and whether it is up and down or all on one level. What I've done for going any distance at all is lay it on a hand truck -- a sturdy one -- on its side. If it's all on one level, one could also use a good big furniture dolly. Strap it on well -- actually, using an appliance truck with the built in strap is a good idea if you have one. Going up and down stairs is quite doable, just ease it down a step at a time. Not a bad idea to have someone tailing onto a rope. Never EVER let someone be below the truck. Going upstairs will be a LOT easier if the hand truck has stair climbers on it. Either way may take the effort of two or more strong people...

5 feet long! I'll bet 4- 500 lbs. easily

From some older posts.