We have rebuilt all the sliding windows on our 1973 GB36. We use stainless track. It's available from Fisheries Supply here in Seattle so it's real easy for us to simply drive over and get what we need. I would not use plastic because I'm not sure how it would fare over time with UV.
The track in our windows--- top, bottom, front and rear--- are individual sections. The windows in the main cabin do not have 90 degree corners because the upper and lower sides of the windows follow the shear of the boat while the ends are vertical.
I cut the mating ends of the track sections so the join at the corners is roughly 45 degree angles. I do not bend the channel to make the corner. I'm not sure that's even possible, and I've never seen it done if it is. I determine the exact angle for the cuts at the end of each channel section by making a cardboard template of the actual window opening angle and then bisecting that to get the angle for each channel end.
If you have questions about the care and feeding of American Marine boat (Alaskan, Grand Banks, etc.) I suggest you join the Grand Banks owners forum if you have not already done so. Grand Banks Owner's Resources
There are people on that forum with a wealth of knowledge about these boats. The forum founder, Bob Lowe, who once owned a boatyard here in Washington that specializes in the maintenance, repair, and restoration of Alaskan/Grand Banks boats, restored an Alaskan 45 from a piece of junk to an absolute showpiece.
The GB forum is a great place to get specific information about these boats, particularly the wood ones as there are a number of people on that forum who have woodies and who do all their own work.