FWIW, there was a discussion for successfully welding that boom on another forum by professional welders.
http://www.shopfloortalk.com/forums/showthread.php?t=1677
I have been a member on that forum for many years.
I didn't read all three pages. The procedure being suggested sounds much like one for cast iron.
They are suggesting nickel rods, Others have recommended higher numbered stainless rods.
I'd try Hastelloy, or another TIG filler I'll have to look up.
You need a gap between the faces, I'd open it up into two pieces following the crack if you can. Cut away enough to leave a 1/4" open root, Something very thick is tricky, you want access to the very root, but a 37 degree angle on each piece leaves you with a lot to fill.
I would use TIG, maybe even AC to "cook" as much of the junk out as possible. The process, after a big preheat, is to puddle in the middle of a surface & get a bond with the dirty ductile. Work your puddle outward as you go until the whole surface is coated. I'd go a bit beyond the whole surface of the break, running onto the surrounding metal a bit.
Maintain preheat all you can, repeat the process with the other half.
Now you'll understand why you removed so much metal, Place the two pieces a pit farther apart than you want the finished product, you want an open root. Weld will shrink as it cools & you'll have to line bore anyway to put bushings back in.
Fill the gap. It'll be hard to get a TIG torch in that deep, you might want stick for that. Peen often with a needle scaler.
Keep it hot after welding.
Good luck!
Meanwhile begin the search for a donor for a transplant.