I have ordered a new Spydercars frame head ("T").
The more I looked at my frame head "T" section the more dismayed I became.
At a glance it was good, but there were some very thin areas and after a good electrolysis cleaning small spots of rust-through became visible. 
Heavy scale between steel layers around the suspension pick-up points cannot be ignored.
It would seem that closing those off in some manner when new might have helped a lot. Maybe some "Seam Sealer"?
At a glance this repair would seem to be a fairly simple task.
Cut off the old T section, grind welds flush, and use the original flanges to alighn the new piece.
Anything that appears simple cannot be, so I have to ask where the complication will arise?
I have chassis drawings, a large flat steel build table, measuring tools and MIG/TIG welders. 
I have copper to back up welds where accessible.
I do not have a spot welder but plug welds may be better.
I bought POR-15 and Penetrol for painting and rust prevention.
I am SO anxious to cease dismantling and begin reassembly!
BTDT (with a good OEM front box from a frame that was tweaked in the rear grafted onto a good rear with a trashed front box...make one out of two).
Print out in as large a format as you can, the chassis diagram available on lotus-europa.com or my archive backup as possible. You WILL need the angles and measurements.
1. Make sure your surface table is level front to back and side to side. some of the angles you'll be dealing with are 2 or 3 degrees. Helps to start from a 0 reference.
2. grind down the welds to break the front box away and leave as much of the longitudinal box untouched as possible.
3. Offer up the Spyder front assembly and set your angles properly. BIG welding magnets help position the box to get your measurements. Don't forget the diagonals (pick the same point on both sides of the box to measure to the same location at the end of the rear fork. I used the upper suspension bushing end to the rear of the fork to square it off.)
4. Make sure your box angle, which sets the fixed caster angle, is right on the money.
5. Tack weld one side top and bottom of same side. Recheck the diagonals and adjust the loose side to match.
6. Tack other side and recheck diagonals and angle.
7. I'd use a MIG bead, not plug or spot welds, to attach the box. across the top, down the sides.
I'm sure I missed something; I did the last one in '09, so the memory is a bit foggy. But, you get the idea.
Stress, though, is on ensuring the measurements and angles are correct. That is most important.
Just my thoughts. Others, feel free to chime in.
