You can look down the bottom of the hull, in both the 'flat' areas between the chines & along the chines to see if there has been deflection. Esp if the transom is soft & has had a motor hanging off it for years, w/out having the transom for support & bunks under the transom to support all of the above, can cause a 'hook' to develop in the hull.
Here's an iBoats thread about a hook in the bottom of an aluminum boat, that should help explain what I meant.
A neglected boat, w/ soft wood, filled w/ water for long periods of time, and not well supported,- would be more of a long term problem then anything you'd do now. But moving around in the boat (your weight & moving around), adding back materials (more weight)and glassing them in to place could all contribute to minor distortion becoming permanent.
My FireFlite will distort out about 4"+ on either side of the hull when I lift the cap off. At 15' long, I am spacing my cross braces at about 5' centers. The
blue in this drawing are the crossbraces @5' apart, that gives me some room to work on the deck between the braces. If it looks like I need more support, I'll add the
red diagonal gussets.
This will keep most of the space between the braces open & add support. my FireFlite has a little bit of contour as the gunwales move from the stern/transom forward. So I too measurements about every 18". It looks like that will give me enough reference when I start the put back. You may need more measurements, or fewer depending on the actual shape of the hull. I pulled the dimensions from the inside face of the hull bottom lip at the cap/hull joint. Depending on how your cap fits onto & is attached to the hull, you may or may not be able to measure from that lip as well.
This is my cap/hull joint: