Lot's of work if you do it right... makes adding crossmembers, motor mounts etc. easier. When working with stressed material that is 40 or 50 years old, you will discover that there are cracks that develop over time. The cracks originate from (link)
stress raisers Eliminate the stress raisers and box the frame and it will be stronger and more ridged... less apt to twist if you add additional crossmembers.
So enough of the technical stuff! Get yourself some cheap paneling at home depot, don't use cardboard. Clamp a strip ( using many clamps) to the frame rail, mark it out with a Sharpie. Cut it out with a scroll saw and transfer to a piece of
1/8" 1018 strip... that's all you need!
The paneling has similar bend characteristics to bending the 1018... so if you see any gaps when you clamp it up. Grind the frame until you get near zero gap. That way you will get minimal distortion when you weld it up
slowly... tack it along the length and then fill in the welds 1" at a time. If you can put your bare hand on it without getting burnt... weld some more.
Link to some pics
Frame box