mate - looked at your whole photo thread an your car would not be allowed on the road in New Zealand. at the very minimum - it needs a k- member for the centre x-member and they are adamant that it has to have that but prefer us to put a double k - i.e. braces running forwards and backwards of the centre cross-member and as mentioned the rear 4-bars are in single shear which would need grade-8 bolts min but once again double -shear ( the rod-ends inside two steel plates) is strongly recommended. I would suspect your chassis is flexing around a lot. the rear x-member doesn't look that strong either!
its a seriously cool looking little hotrod but I suspect you've got a lemon that will take a lot more money to sort out.
sorry!