I do not recommend either front or rear GD uprights for use on a GTD40. Their failure induces heart attacks in passengers? But clearly good for GD products.
We have a local customer with an early CAV GT tube frame car. Since it's sort of CAV, sort of GTD, there is some parts interchangability. The rear upright castings were drilled oversize for the clevis pins, one had a bearing bore so loose the wheel bearing almost fell out, plus there were hairline cracks around the through bolts for the suspension mounts. Scary...
At first we tried to fit stock CAV parts, they would accept the axles however the brake mounting points were different so we made a custom set of uprights based on our proven fabricated steel uprights. Needless to say the increase in confidence in the car is immeasurable. The rear end geometry is also improved.
Could you send some pics of your uprights to me at [email protected] please? We'd be happy to work out a solution for you. Our steel uprights weigh 5.5kg each by the way, the same as the stock CAV aluminum castings and mounting blocks.