From an engineering perspective no need to stagger or angle cut. A straight cut is fine. Just be sure to have a welder that is certified to weld out of position do the welding. Also, fishplating isn't neccesary if the welding that is performed is full penetration and good quality. Boxing the open side of the frame in the area being spliced would add more structural integrity than the fishplating.
Since your in Texas finding someone with a welding rig to do the welding should be easy enough, that is unless you are a certified welder, if so my hats off to ya.
Hope this helps.
John
