A choice of gel coat or epoxy is not the big issue. That is whether you want to use polyester or epoxy. Gel coat is just thickened polyester with a pigment.

I used the West Systems Flex Epoxy, thickened with colloidal silica, last time and it worked great!

Make sure to clean and rough sand the area you want the material to adhere to.

I tape and cover where I do not want product to end up, including all the way to the bottom of the boat. It can get a bit messy the first time, as you will probably mix too much material.

Put wax, I use HiTemp Mold release (because I have it), in the thread holes as well as waxing the bolts. Be patient and put several coats of wax on the beams, waiting for them to dry in between and buff each time. I like to make sure with about 7 applications. This will make sure it is all covered.

Make all level and get ready with a tape measure to use crosswise. May be issues, but I have not had any with F18s.

After reapplying the beams snug the bolts down pretty good but you do not need to crank until you hear cracking. The filler will be your new bed.

Caution should be made if you think there will be closing around the beam (negative draft). Keep in mind that one day you will want to take the beam off, so it has to be able to release.

Have a fun time with it you will like the results,
Dan