At the time I did the first run of basses, Thunderbuckers were a pipe dream, the routes are probably 3/4" deep to accommodate the SDs. You might need a 1/4" (6mm) wood shim about 2" (50mm) long. Approx 3/16" below the strings is what I install them at.
Thanks Carlo, that sounds easy enough for a woodworking noob like me
The strings should only be 1/2-5/8" off the body absolute maximum at the bridge, how high are the saddles? Is the neck shimmed? Action really high?
The action is fine, exactly how you set it up - I haven't touched the saddles or shimmed the neck. My 1/2" estimate might be a bit off - I've been living in the metric world for 25 years LOL.
I'll put in a timber block as you suggest and will adjust the pups again after that. It's all good - the pickups look and sound great, but I can't get enough bite/resistance from the pickup height adjustment screws at the moment to adjust them close enough to the strings.