Just a couple of suggestions for you-
- dsraven's guide coat suggestion is a good idea if you're not doing it. Get the powder type. This is the one I use- little pricey but saves money and time in the long run.
https://www.amazon.com/Mirka-9193500.../dp/B00GTYHCGG
- dsraven's suggestion for a long board is good too. I have Durablocks, but my favorite is a piece of balsa about an inch thick and 12" long. For smaller detail work I usually just cut up a paint stick or find some object in the shop to fit the contour (I have a few brooms with short handles now)
- I like the Featherfill G2 spray-on polyester filler. I usually go through a few rounds of spraying-blocking with 80 grit before coat of epoxy primer to make sure things are perfectly flat
- Looks like a MarK bed. Now's the time you want to decide if you want all the spot welds to show through the top coat. If you don't, you should fill them and block them flat, followed with some etch primer then some spray/block with Featherfill G2
- The two rivet heads in the middle of the rear fender tend to move around and crack the top coat. You might think about removing the center brace and welding the holes shut to side-step that future problem
- Now is also a good time to decide if you're going to use fender welt between the bedside and the fenders. If you are not going to use welt, it pays to give some attention to the bolt flange/fender edge on the fender to make sure it is crisp and flat and the gap is even. The MarK bedsides are pretty flat, but a fender with a wandering flange will make them wavy when you bolt them up
- Your wheel tubs look very similar to mine with a flanged edge. You might want to check where the carriage bolts go through the bed and make sure the lip of the carriage bolt head doesn't interfere with the tub flange.