Sorry about not getting back sooner on this post.
As has everyone, I have been pretty busy, it`s pretty hectic around here!
Welding stiffeners {gussets} in solid {all the way around} creates what is known as stress risers {a place where a crack can form}, normally on or next to a weld!
The stress comes from any lateral or parallel movement of the base material within the weld zone against the stiffener. Skip welding the stiffener leaves an open space in between welds. These open spaces serve as stress relief points {a place for the stress to dissipate}.
One should take great care in welding, each bead must not have what is known as a weld crater at either the beginning or end of the weld. Any crater in itself is a stress riser.
Wrap the welds all of the way around any end of the stiffener and weld any inside corner too. 1 - 1 Weld one inch skip one inch or 2 - 2 or 3 - 3 ECT. {Doesn`t have to be exact!} The idea is to leave someplace for any stress to dissipate.
My choice of filler material for the welds would be Er-70s-1 for clean new material s-6 for older material {Mig wire} or ER7018 Stick electrode {D.C. only} or A.C. stick electrode machine use ER6011.
Try to get the weld zones as clean as possible before attempting any welds. that means removing any and all mill scale, rust, paint, grease ECT.
If you have little or no welding skills, the welding procedure on this type of work may be best left to a professional. Because it is imperative that no craters be left at the ends of any weldment.
If the welding is done properly changing the molecular structure of your frame around or near the welds should not be an issue. I could go into all of the Techs. of how and why and all about carbon precipitation, H.A.Z.`s {heat affected zones}. and all of that but most here would not understand anyway so I won`t bore all of you with that information.
I hope this helps you