Hi Robin
On the last KVA I restored the front ball joints used were Granada MK 1, remember that the KVA was sold with only a guide as to what parts were fitted/scourced for the build, thats why you are unlikely to find two cars of identical spec.
Replacement joint gaiters are rarely stocked by factors but I have found suitable gaiters for sale at autojumbles
Water sealing at the front end, this is as you say tricky, as on most 40s,make sure the scree base is well sealed to the bulkhead top panel , make up some seal carrying panels at the A post area, form a drain pipe/chute at each side of the screen base channel, seal the bulhead
front panels and pass all pipes,steering column and cables through good quallity grommets.
This should keep water ingress to a minimum, water entry to the area under the nostril from my experiance ,make drainage holes ,water will always find its way here via the nose intake.
To strip and start the build again is a wise option ,you can be sure everything is done to your spec, and build in the current test requirments.
Take the opportunity to fabricate removable engine access panels in the engine bulkhead, make provision for the sealing of panels around the tanks and filler tubes,
cheers
John