The "T" extrusion looks good and is certainly strong. Faster mentions making sure that the mounting holes line
up with the existing holes, which is a good point to help avoid leaks from unused holes. Using the same holes will be impossible if the flange that goes onto the deck isn't deep (or long) enough to reach in to where the existing holes are. Better measure. Twice. Depending upon the extrusion thickness, the "T" may also be more difficult to bend than an "L" There are lots of sources for rails, and Taco is not the only supplier. Based on the pictures of your boat above, it looks like you could start attaching the new rails at the stern and use the leverage of the rail itself to torque it into position, drill the next hole, caulk & fasten it, and so on. (It looks like the most curve is towards the stern, so that's where the leverage of unattached rail would be most useful.) Figure that it will take about three times longer than you thought to do this, because it will be better to do it right, once, than three times.