My 78, k5 blazer 4x4 with 4 inc lift, recently swapped in 1 ton axles and 35s and 400ci v8 has grown a crack.  I saw the crack today in the front crossmember which ties the engine mounts toghether, the crack is close to where it mounts to the frame.  My question is is this common in solid axle chevy of the 73-87 era>  Should i just weld the crack up and weld some plates over it also and im guessing that the other side of the crossmember will also probabley start cracking, so do i just reinforce my existing one or what?  Also this makes me worry about my frame where the steerting box mounts, im decideing wheather to box the front section of the frame where the steering box mounts and put some longer mounting bolts to strnegthen the frame or just put in a bolt on steering box brace which costs $ while welding for me is free?          

Yes, on a 73-87 stock frame the cracking is somewhat commom when wheeled hard.  You could weld the crack and reinforce the cross member with plate steel or you could bolt in an after market cross member (Offroad Design has a nice one).  I would bolt in a steering box brace.  It wouldn't be that hard to build your own steering box brace.  You could go to ORD's web site and look and what they have and build your own off there picture with measurement off your frame.  

there are/were preformed, weld in frame strengthening plates for the steering section. all of my fullsize truck crossmembers have cracked near the rivots at one point or another.  

The reason it is cracking is from too much counter torque on engine/tranny combo. Every ounce of twist placed on drive shafts it applied in the exact opposite direction on engine/drive train mounts with the same amount of force. When you do not properly gear axles for tire size and lift and try to make up for it with granny gears and low range you place a LOT of twisting stress on frame which can lead to fatigue and failure over time. The PROPER way to fix this is to first drill stop the crack then notch it out a bit and weld it so you get full penetration of weld. Next, change the gears in axles!!! You need to reduce the counter torque forces and remove the cause of the crack rather than band aid fix it and force a stress crack somewhere else. Rather than treat symptoms, you need to address the cause of the problem to begin with.