Best way to remove all old hacks and clean database of left over junk?

[Quarterbore]

I am preparing for a major updrade of one of my sites and I would like to make sure I have removed all the unused database changes but I have a fealing I am setting myself up for failure.

My site has been live for a long time and I have kept up with all the various 3.5.x and 3.6.x updates and I have installed and removed hacks along the way but there are database tables in there that I really would like to get cleaned up. My problem is, I still have hacks that I need to keep added including vbcredits, vbclassified, photopost, vbadvanced links directory, and others.

I thought about using IMPEX to import the forum data but I am pretty sure if I do this the userids will shift up for the rare case were useres were deleted and that will make transfering over other tables a bloody nightmare as for example items originaly posted for user 2345 will be assigned to 2345 but that user is now user 2300

Is the best way for me to do this to install a fresh 3.7.x forum and look at the database structure and then go through my database and tables to find things that were added but never uninstalled and then clean from there and just upgrade using the vbulltin upgrade scripts instead?

I assume that is the proper way but with an old board with the possibility of quite a few of my own wacky hacks it is going to be a fun project to get this site cleaned up so I figured I would ask if anybody has any tricks they have learned to address these issues.