Henry, your plan is almost identical to my set up, except I use an external return pump and skimmer. Methylene chloride is the main solvent in weldon #3 and #4, but these products have acetic acid and or MMA resin to slow evaporation and thus strengthen the bond. Using straight methylene chloride to solvent weld cast acrylic will probably result in weak seams with a lot of bubbles. I've heard the best solvent is a product called MC bond, which unfortunately is only available from the manufacturer in gallons. You could use the meth chloride, but try adding some acetic acid to slow the reaction down. I have no idea how much. My suggestion is just buy some weldon#4, it's not very expensive and available at any plastic supply house.