As I wander through my process, I come up with questions. I'll record them here and record the answers once I find them.
Q: Would a baffle in the hydraulic reservoir improve performance?
Q: Is NPT the right threading system for our connectors? This source suggests that UNO and BSPP are better for systems that will be disassembled and reassembled.
Q: How will I attach the compressed air to the tank for my leak test?
A: Tom explains his answer here. It's a good one.
Q: Do I need to paint the inside of the hydraulic tank?
A: No. Tom and others agree that it's a bad idea.
Q: Is the intake filter on the hydraulic reservoir harmful?
A: No, the source that concerned me also explained that case drains shouldn't be filtered on return into the system, so have to be filtered on intake. See here.
Q: How should I clean the inside of the reservoir before welding it closed.
A: Scrub down with Scotch Bright and then wipe with Oil (I used WD-40) to protect it until the tank is in use.