Welding cast steel to maleable steel?

I don't imagine that beam is cast iron. It's probably a forging. If that is the case you can join the beam to the carbon steel from your spreader using a low hydrogen process. A tensile strength of 70,000PSI should do. If you have a wire welder, that is a low hydrogen process in itself when using a cover gas and hard wire.

I know where you can obtain some excellent filler metals if needed.(^:

Some things to try are NiCrFe and Esab 6IMP rods. (was hoping Steve would have chimed in here) Pre-heat to around 400F and hold it there for the duration of the welding. Run the rods as low on amps DCEP (reverse) as you can just enough to keep them from sticking to the work. Run short beads not more than 1/2" to 3/4" at a time depending on how thick the metal is, thicker will allow a sligtly longer bead. Don't let the base metal get too hot from the weld. Allow to cool slightly after each segment of weld is applied to try and maintain no more than 600F overall. Hard to say how well this will work but a V joint will help gain strength and also slow cooling when done (1.5 to 3 hours is usually sufficient time to slowly reduce the part temp from 500F to 150F, from there it can air cool without a problem)

You may also want to try a standard 309 or 316 stainless rod as well. Sometimes these will be sufficient and sometime these welds just will not take for nothing. One other thing you may wish to consider is bolted joints using a steel cover fish plate on either side of the joint. Sometimes this will be the best choice and save you a lot of needless costs and time.