If I was you, I would do the following:
If you are pulling the wiring harnesses out to do a tuck and so then pull all of the wires out while you don't have a motor and you have less to worry about when degreasing. Once the wiring is out, spray the s*** out of it with degreaser and hit it with a power washer. Most degreasers say to let it dig into the grime for about 10-15 mins then rinse. If there is nothing in the bay, I would just soak it, let it sit for 10-15 mins, and if its really dirty, hit it again and after another 10-15 spray it out.
If you are leaving the wires in the bay, you don't have to remove them to degrease everything. All you need to do is buy some saran wrap from the grocery store and cover all of the wires. Wrap the fuse box and anything else with wires going in it really well just to insure no degreaser gets in. Once tht is done, follow along with the same degreasing method.
Once everything is clean and dry, go over it all to make sure no grime is left over. If there is anything left, scrub it with either a scotch brite pad or a brush and wash with soapy water(basically common sense to clean the left over grime).
I like the POR-15 but thats just me. If you go to their site,
www.por15.com, you can get everything you need. They have cleaner called marine clean and metal ready. You spray the marine clean on and follow the directions exactly. Once that is done and dried, you spray the metal ready. After the metal ready is done and dried, your ready for the paint. You can brush the paint on or spray it. I have brushed it on and it has great leveling so you wont really se the brush strokes after it settles and dries.
Note: I believe you can lay the por-15 right on top of the paint as long as you use the metal ready and marine clean but you should check with the directions first. You may need to hit everything with a red scotch brite pad(available at home depot, walmart, autozone, etc.) to scuff the paint so the new stuff has something to ahere to. If that is the case, you SHOULD scuff it before you use the marine clean and metal ready so there is now left over paint dust waiting to make your new bay dirty...hope this helps