you might find that the mortar tubs are a little shallow but they will still probably work for something anyway.
Beware, it isn't so easy to cut a larger nice round hole after you have already cut a smaller one in the same spot (no place to put the center bit on the whole saw to keep the whole saw from moving around as you cut, you have to get a bit creative to manage to do it.

However, take a tip from the barrel ponics manual, if you cut the hole just a tiny bit too small and then warm up the plastic to thread the fitting through the plastic, you may manage a leak free seal with just a little bit of aquarium silicone. I would probably use the regular drinking water pipe pvc fittings for this instead of the electrical conduit fittings just because I don't know if the conduit is food safe or if the pipe sizes are interchangeable between the two.

And for when you go to get gravel. 1 ton of the 1/2" brown river rock might fill aprox 160 gallons of grow bed. 6 tons might be around 950 gallons of gravel. (This is based on the pebble junction estimation that it takes 1.25 tons to make a cubic yard of gravel.) I have gotten 2 separate 6 ton loads of gravel delivered here. 6 tons is the weight limit on the delivery trucks that pebble junction uses for residential deliveries.