First up, I bolted in this support for the back of the driver's seat. The seat bottom will bolt to brackets on the floor; this supports and constrains the upright back section. I need to take it back out again, of course, because I need to paint the steel angle it's bolted to.
The donor car, like most passenger cars, had more braking up front. The kit, being rear-engined, needs more balanced brakes, so the big discs and 4-pot calipers on all four corners will help, particularly since the brakes are unboosted. There will be a proportioning valve in the hydraulic system to tune the balance if needed.
I'm thinking axles and rear suspension might be next.