Help and Advice sought on migration of heavily hacked board

[Michael Morris]

Hello all. Family troubles, school, and so on have kept me out of here for awhile, but I'm getting ready (sometime tonight) to renew the vb license of my site and begin work on migrating the monster to 3.5

The site uses the following hacks either linked from here or hosted here.
vbadvanced CMPS
vbadvanced gallery
vbadvanced links
Secure Downloads

It also has a custom review script and several miscellaneous hacks running about. My goals in the upgrade are to segregate the code of each of these away from Jelsoft so I can stop hacking my Jelsoft source code files completely (which is turning into a pain). Secure downloads has independent code, but needs to be hacked for PHP 5 support. I don't know how far along Brian is on getting his products up and running on 3.5

My board is so badly fragmented by my inexperience and old hacks that I have no idea what they are supposed to do that I am STRONGLY tempted to use Imp-Ex to transfer the core boards materials over and then reconstruct the gallery information by hand copying those appropriate tables from phpmyadmin. Obviously I am going to do this process on a test server before trying it on the main server.

If Imp Ex does an import will it preserve the old thread, post, and forum ids? If not I can't use this approach since I have several tables that are relationally keyed off post and thread id's and these must remain unchanged for those posts to work, and I'll have to extract that data through seperate passes.

I truly want to clean my data while doing this upgrade along with my codebase. Any hints/tips/examples out there

(By the way, this is a board with close to 3 million posts).