Cut off existing mounting points and brackets
The mast had existing mounting brackets welded in places that aren't suitable for my boat. I cut these off using an angle grinder with a metal cutoff disk. It was very quick cutting through the aluminum...Careful not to take off too much! I also took this opportunity to sand off the existing paint and smooth out the overall surface of the mast.
Create and install mounting bracket for GPS receiver
Using a combination of a bracket that was on the original radar mast and some aluminum plating that I had laying around, I created a nice little mounting bracket for the GPS receiver. Basically I just formed the plating into a two piece circular mounting bracket using a piece of 2" pipe as a form.
Prime and paint
I used automotive aluminum spray cans to prime and paint the while unit. The paint seems very durable. We'll see if it holds up.
To install the mast I used a 3x6 (1/4" thick) piece of aluminum as a backing plate and a pivoting bracket at the base. I drilled holes to accommodate the 1/2" stainless bolts that would secure the mast to the backing plate. I applied silicone sealant to the bottom of the mast and around the screw holes to prevent water leakage.
Using more spare pieces of aluminum, some bits and pieces from other clamps, and a hose clamp, I attached the mast to my aft safety rail. This is still pretty ugly and I need to come up with a better clamp here.
I quickly realized that the mast still did not have enough stability. I created an additional diaganol brace from my GPS mounting bracket to the cabin deck on. I used a piece of 1" aluminum tubing for the brace. I think it could still use one more brace on the other side but I'm going to wait and see how it performs first.
I installed the radar and GPS receiver using the provider hardware. I did realize, after the fact, that the hole that I had drilled for the cabling wouldn't accommodate the GPS connector. I had to make some last minute modifications to get it to fit.
If more detail on any of these steps would be useful to you, ask for clarification in a comment below and I'll update with the further detail.