bolt in rollcages

[3MCRacing]

you'll never see a strong roll cage mig welded, welds are too weak.

Mig welding has been used in race car chassis for many many years. it is more reliably done than Arc and less expensive then Tig that is why it is used so much. Look at any race car from class 11 to SCCA Trans-AM and i gaurantee that you will find Mig welds. When using a bolt in or weld in cage the main hoops need to have bases that will spread the load. ie... if you weld a 1 1/2" tube to the door jam where it meets the pan (not on the pan) then any impact will be spread out in the same dia as that tube but if you welded that same tube to a pice of 1 1/2 " x 3" flatstock then the impacting load has a much larger area to spread. If you really look at Rally cars you will notice that the tube going to the firewall does not end at the firewall it extends through the firewall usually to be used to support the suspsension mounting point.

And just in case you didnt know "monocoque" is an engineering term indicating a structural shape where loads are borne on the skin of the structure).