You might want to look into Chassis Engineering's crossmember replacement pieces that restore strength to the removed parts. You have the holes where the original stamped steel side went, so find some replacements for them(which CE might have listed) and bolt everything back in to get your geometry back to stock parameters. I'd also leave the stock rear crossmember in the frame and use CE's parallel spring kit, which is a bolt in, and in my '40 PU I put a Ford 9 inch into the springs, and just ground the lip of the crossmember to be sure it cleared the 9 inch. Keeps everything strong back there, as the rear frame end crossmember is not strong enough alone. boxing the frame is a good move, for the strength to keep from fame flexing (which Ford made the frame so it would flex) and cutting and welding it ruins that, so better to make it so it no longer tries to flex, and prevent cracks in it where it no longer can. Also check-out CE'sblot-in front crossmember for rack & pinion steering (Mustang II). It works really well, and has adjustable upper spring pockets to front height. Their stuff works really well when you follow their fabrication instructions.