Here I was about to begin flaming you to a crisp for a new install of a 3+ year old version, when it looks like the board really has been around that long... Oops.
Take a peek in the server error log - it might provide more information about the 500 error that you get in the control panel.
Twiddling the signature checkbox shouldn't be too hard... IIRC, all the new topic/reply form HTML is in postings.cgi - finding all the checkboxes inside should be easy. The signature ones should be plainly marked (as name="Signature", probably).
You should really, really consider moving from the 5-series to the 6-series, or perhaps to UBB.threads. That's three year old code that you're running. There are many known security and stability issues that you certainly don't want to get bitten by.