On my harp guitars I cheated: I did not have the usual top extension up the harp arm. That made the top into a 'normal' guitar top, more or less, and you can figure out the thickness and bracing for it in the normal way.
The first one, which I call a 'true' harp guitar, has the harp string pulling upward (or, in this case, downward) on their soundboard, which is perpendicular to the normal guitar soundboard. The harp strings run across the guitar top, being stopped at a bridge along the bottom edge of the lower bout, and the tuners are on a 'sponson' attached to the wide tail block. There is a post from the neck block to the tail block to support the tension of the harp strings, and the upper end of the harp soundboard is attached to the guitar headstock at the top. I know this is confusing, but there is a picture of the beast in the latest American Lutherie, and there was also a back page layout in 'Acoustic Guitar' a couple of years ago, and they will make it plain. On that guitar the soundboard only took the 'normal' guitar string load, and was simply designed and 'tuned' like a normal guitar.
My second one was a bit less unconventional, except for the fact that soundboard did not extend up the arm, as I said. In that case, the harp strings are attached at the guitar bridge in the normal manner, with their tuners at the top. Given the larger string load on the top I made it thicker than I would or a six-string guitar, and more like a 12-string. The bracing is also heavier. Ghe basic idea there is to use the 'cube rule'.
We like to make tops as thin as we can, so they'll be light enough to be easy to drive with the minimal horsepower of plucked strings. What sets the lower limit on the thickness is the stiffness of the top, which has to resist the torque of the bridge over the long term. That torque is a product of string tension and the height of the strings off the top at the saddle. Given the tension and height, it should be possible to calculate how thick you'll need to make the top.
If you assume that all of the top wood you use is 'the same', then all you need to do is keep track of the guitars that worked, and how thick you made the tops, and you can figure out how thick to go for something different, provided you don't change things like the outline or body length too much. You just use the 'cube rule', which says the the stiffness of the top varies as the cube of the thickness, and ditto for brace height. If you used a top that was .100" thick with braces .5" tall for a successful six string, and you're making a harp of the same basic size and shape from the same wood, then you just need to know the string tension. Suppose it's doubled: then the thickness for the harp top, and the bracing height, would be thicker by a factor of the cube root of 2, or 1.26. I know it doesn't seem like much, but it works.
This assumed that the wood was 'the same', which it never is. To be really sure you need to measure the 'Young's modulus' of the wood to correct for the differences. It's not all that hard, but I won't get into it now. You can also make use of an interesting property of soft woods, which is that the Young's modulus for bending along the grain is proportional to the density. Two pieces of softwood with the same density will have the same stiffness at a given thickness (within pretty close limits), no matter what species you use. Just keep track of the density, and use the cube rule to help you copy something that already worked.
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