350#/in springs are going to be pretty sporty, and may be too much for the Tokicos. MM has some good tech on matching dampers and spring rates (either conventional springs or coil-overs). The good news is that coil-over springs are easy to come by in a variety of rates, so you can experiment.
I didn't personally install the coil-over sleeves on my Mustang, so I can't speak to how tight the sleeves were to the strut body. On my Subaru, when I did Ground Control coil-overs sleeves way back when, I did exactly what you suggested - I wrapped duct tape around the strut body to make the sleeves fit tightly. (As I recall, the sleeves came with large o-rings to put around the strut body to accomplish the same effect, but to me it made more sense to use a wider surface, so I did the duct tape trick. Worked just fine for years of daily driving, auto-cross, and open track use.)
He's already picked up a Griggs, which should be just fine. Concur with adding the lower K-member brace across the back of it, regardless of what brand one goes with.
The advantage of the thru-floor SFCs is the vertical component - that's what gives them the rigidity. Think I-beam. I don't know if the KB setup provides the same rigidity or not, but it uses a lot more metal to do it, and (as already pointed out) the KB kit won't fit the longer F/Z wheelbase. Whether the thru-floor setup is the way to go for a street car is obviously debatable, but one advantage (other than the greater ridigity) is that you can make them with a straight 2x3", whereas the under-floor type have to have some bends to conform to the underside of the car.
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