I personally like the idea of weldon 40/42 tanks as opposed to solvent. Weldon 40 is roughly 2x stronger of a bond compared to a solvent bond if done correctly, given 1 week to cure. Annealing the bond increases strength to almost 3x that of solvent. Similar results can be achieved by using a vaccuum chamber which is what I am currently working on doing for a very large custom build. To properly use weldon 40/42 it’s suggested to cut the acrylic on a low angle of roughly 5 degrees. This allows for adequate amounts of 40/42 to bond the two panels. Making the 5 degree cut is hard to do yourself. It should be done using a CNC router with the correct bit. Being a machinist I feel you could do it. If you have ever seen a large acrylic tank with 1” material or thicker, it’s likely not a solvent seam. Normally a 2 part is used such as 40. From what I have seen, most builders do not follow the 5 degree cut suggestion. Some create a small 1/16” gap between the two joining pieces by raising the acrylic sheets with chain blocks. The gap is then filled with 40. Others just lay 40 down like silicone and place the sheet on top. I personally would not go with the raised acrylic method. I cannot advise for or against pocketing the acrylic as I have not tried to do so.
If you go with solvent, use the pins method as mentioned above. Weldon 4 works well but a custom blend of acetic acid (99% pure) and methylene chloride will often result in a cleaner joint.
Joey in the video gives a good demonstration on what to do. Some wait 30s-1min before pulling pins while others pull immediately. I personally wait the 30-1min as I like the molten acrylic fillet that helps seal the edge upon pulling the pins.
For drilling, I would look into overflow boxes. Lots of cheap ones available on eBay. For myself I would want a minimum of 2 1.5” drains on a 300. As mentioned the bigger the sump the better. Make sure the sump can handle the added volume of water from the tank in case of overflow. This would be the normal level of water in the sump + an additional 1” or so of water until the level drops below the height of the overflow teeth. For your tank that would equate to roughly 12 additional gallons (assuming water level kept 1” above the overflow teeth).
