I built the main frame section over my holiday break at the end of the year. Square and level. Actually, having a square and a level meant I already started with more tools than the average large scale, spec home builder
I am lucky enough to have access to engineering tools at work so I was able to perform various analyses and optimize tube locations and actually reduce the weight of the cage by quite a bit from my first cut with a fair amount of triangulation "by eye".
Attached is the analysis model of the proposed chassis design. The cage in the rear will house the sequential 5 spd transaxle I have and the engine will be set back about 6" to get nearly all of its weight behind the front wheels.