1/8 is definitely better than nothing, and along with welding the cracks, will be stronger than original. I am not sure how big of a welder you have, I wouldn't do the job with a typical homeowner duty 110 powered MIG, Ive welded with flux core before, but most of what I see in flux core is meant for a person in the home garage making those birdhouses out of old license plates and is barely adequate for something like that. if you have (what I'll call) "industrial duty" equipment with at least 0.035 wire you'll probably do alright with flux core. If I had to work outside, I'd probably stick weld that with a 7018, which, too, is a flux coated electrode. It isn't the flux that bothers me so much....
If I could get at least the front 1/2 of the truck thru my garage door and out of the wind (you haven't seen my garage!) I would consider pulling out the MIG (I have a big mig here at home, 220v powered Miller) and doing the job with that.
be sure you clean the parent metal GOOD on both sides, (wire brush, needle scaler if need be, ideal would be to blast it but not always practical) and that the plate used, goes at least 1" past the outside edges of all mounting holes. and go from the other side of frame and re-drill the steering box mounting holes/ With only an 1/8-3/16" plate you can probably get away with the original steering box mount bolts.