Vintage or authentic Thunderbird pups are a very thin twin coil design that doesn't really need the deep rout of the contemporary version. When I retro fitted my basses with Mike Lull's I went to the local nice wood place and got some Mahogany stock in 1/8" and 1/4" thickness, I shaped and glued them as spacers in both routs, thus I could still use the stock screws. I also slipped some dense foam underneath and used some springs around the screws to help create additional tension.
Here's a Lull pup.
Maho block
Without blocks
With blocks
Done, but could probably use a second black spacer underneath the the bridge pup ring. Mind you, the bass was perfectly good to begin with, it's just my own chrome insanity that made me start with an $1800.00 bass, take all the hardware odd and replace it..........