This is not my area of expertise, but I'm wondering where you live and how deep the frost line is. I'm in Ohio, where it's a lot deeper than 18".

Engineering aside, 50 linear feet of 10" x 6" footing is three quarters of a cubic yard, or something like 60 bags of concrete. Even with a short-load fee, it might be cheaper (besides being a whole lot quicker and easier) to get the concrete delivered ready-mixed.

The crew that formed and poured for an outbuilding on our property framed 2x4's 16" o.c. and skinned with 1/2"osb. The ties were placed 6"from the corners and where the forms butted, and every 16" in the field of the forms.They used 3/8" rebar. After the footings were poured, they formed the whole stemwall and poured the stemwall complete.If you can do this, it would probably be a better pour, as pouring sections might result in weak joints where the existing and new come together. Have you considered block foundation? Looks great,you can work it at your leisure, and will support the weight of your structure easily, although i would still fill each cavity with crete.Jon's advise of getting ready mixed is great advise as a stemwall would be hard to keep up with using a mixer ,especially if your flying solo!

Thanks for the input. I live in a temperate area and code only requires 18" depth. My location is a place where you can't get a concrete truck to, so the total charge for concrete, short load, and pumper is about $1000, which is why I want to try it myself. As for concrete block, that's an idea that I'll consider.