I was given a Saga Tele kit some time ago that I keep putting off because I don't know how to do what I want to with it.

Here's the basic scoop: I have a normal tele and a lot of other guitars already, so I want this one to be something different. I have a Bigsby B50, an extra ashtray bridge, some vintage Fender style tuners, and a set of Filtertrons looking for a home. This kit guitar should be that home.

I don't quite know how to get there though. Here are my two big hangups that I need help with:

1. I need to fill in the bridge pickup cavity (and possibly the neck pickup cavity depending on how things line up) and reroute it for the filtertron as I won't be using pickup rings because I don't like the way they look on a tele. I understand that the easiest way to do that is route out the existing cavity into a standard shape (square) that is easy to cut a block to match. I was told to use a wood very similar to the body wood for this. The body is basswood. I can get basswood at Hobby Lobby or any other hobby store, but I don't think it's really very similar to the guitar body wood. It's difficult because the body comes ready for paint with a tough poly sealer on it so I don't know what the wood itself is really like. The sealer is basically bulletproof, so removing it isn't an option. So, what should I use to fill the cavity before I route it back out for the filtertron and how should I prepare that wood (I should be doing something to prepare it for finishing much like the rest of the body with the poly sealer, but I don't know what to do)?

2. I need to keep everything centered properly on the guitar (new bridge, pickups, and bigsby), but I have no idea how to do that. I know this is a really basic question, but I'm not sure how best to go about it. I see a lot of people go wrong on this part with kit guitars, and I don't want to put a lot of effort into this to find out that my finished product is crooked.

I'm good at setup work and I take my time to get things right on projects like this. I should point out that I'm new to guitar building and fairly new to woodworking though.

Thanks for your help!
D