Outer Limits 4x4 Board

Hello everyone, I am new to this site. I am relatively new to 4wding. I have a HJ60 which i am constantly trying to keep alive, and i am rebiulding a HJ45. I have a donor 60 series which i am in the process of grafting all the running gear and motor into the 45. I have to move the front springs out and add a crossmember for the gear box. I have the 45 chassis sitting in the shed at present and have noticed a bit of rust and broken welds. Can anyone tell me what the best way is to repair the chassis? Some areas are striaght forward but there is a spot where the rear spring mount joins the chassis it is riveted and there appears to be a backing plate on the inside which is heavily rusted. Should i be removing the whole lot and placing a new piece of metal and then bolt the mount back in position? Also due to water being trapped in the chassis the bottom of the rail is thin in places and the rivets aren't holding on by much. I was thinking of repiaring all the rust and then maybe doing a few tack welds in the spots lining up with the rivets. Also while i have the chassis bare is there anything else i should do before the rebuild? Any help would be appreciated.
Andrew.