Concrete footer question

I am making a footer for the base of my spa. I need to make a 7' square footer, and have decided to use cinderblocks since I have so many of them. I dug the trench, and now am trying to get them all level. It is important for it to be level since the spa will sit on top, and I want it level. The outer walls of the spa will rest on the cinderblocks, and then on the inside, I will fill it with peagravel, which will actually support the weight of the spa. I will be filling the vacant spaces in the blocks with concrete, an figure I will drive some rebar stakes down into the ground in each cell.
I am having a dickens of a time getting all the blocks level and straight. Then I had an idea. Would it be easier to run string lines all around the outer edge slightly higher than the blocks, then pour concrete in, and wiggle the blocks upwards until they are true to the string? Keep working them until the concrete sets enough to hold them fast? Right now, I have been around the 28' length of the footer, lifting, filling, tamping, and tweaking, and then getting back to square one, and it not being right. Yesterday, I just walked away from it in frustration, and a little later, the idea of pouring the concrete and floating the blocks came to mind.
The cinderblocks each have a small kickout and indentation on the ends, and interlock slightly. I will be using sack concrete, so will be going slower then having a truck pour it. This doesn't have to be PERFECT, but it does have to be close to keep the spa from sagging or cracking.
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