Hi Brent. Have you thought about trying to shore up the existing rib tops rather than cutting them off? If the main issue is cracks and holes from nails and gunwale screws, you can make surprisingly good repairs - without scarfing - on ribs that appear to be in pretty rough shape. Remove the fasteners and tape off to keep epoxy from running down the ribs or contacting the gunwales. If the ribs are at all punky, you can apply a low-viscosity epoxy like CPES which will soak into the wood fibers and solidify them on cure. Then repair the cracks and holes with regular epoxy. For larger gaps and damaged top edges, apply some epoxy thickened with wood flour. You might add some side-to-side clamping pressure to bring splits back together. Build slightly higher than the original top and then you can sand down to a nice, flat surface.
This may sound like more work than scarfing, but I personally believe the end result is stronger that a scarf because you'll have continuous grain throughout the rib (no reliance on a cross-grain glue joint). Plus you won't have all the scarfs to to see. There are multiple ways to approach any job; this is simply my personal preference for this kind of job. That said, of course I've done and will do scarfs when this kind of repair is insufficient. In fact, tomorrow I'll be scarfing some ribs where the tips are completely rotten away - there's nothing left to repair.