In your photos there is a shot of a weld that is right beside the steering gear box.
This is a very dangerous spot to have a crack! However you are in luck!
There is a kit that you can get off of eBay to re-enforce this spot. Given that you have a crack there it's now a must for you IMHO.
https://www.roughcountry.com/frame-support-brackets-5040.html?gclid=CjwKCAjwxOvsBRAjEiwAuY7L8kZrJCuC1MWuEIb079OMrc1qi11ITiTuDBC-KKaQlLKOnfkerWXPnBoCx2MQAvD_BwEI know it says "4X4" but if you read the fine print to the far right is also says "C10/K10 Suburban" I'd personally HIGHLY recommend that you get one of these kits and weld it into place.
I'm not a welder but many of my friends who are have told me many times that when you have a crack in steel you need to drill a hole and the end of the crack (in the middle of the steel) to stop the crack cold. Welding it might fix it but cracks have a nasty habit of getting past the ending point and past that new weld. Just beware...
I'd also grind down your welds and get some steel of the same thickness and plate over your cracks with a diamond shape plate. This will add extra strength to re-enforce your welds.
Hope this helps and best of luck!