Jay, I do run Moroso rim screws on the Fairmont, mainly because I had already drilled the Centerline wheels for the screws years ago when I did run tubes. (I have never seen any issues of air leaking from the screws). I don`t use screws on the Mustang, as I never drilled the rims for them. My Mustang is mainly a NHRA Stock Eliminator car, running mid to low 12s, with low 1.60 60 foot times, but even with the 331 cube bracket engine, I am using the same wheels and slicks, with no screws, 60 footing mid to high 1.3s, dropping the clutch between 6000-6600, with no issues. I think the fact that I mount the slicks myself, with no tire lube on the beads helps in that respect. I put a small White Out mark on the slicks sidewall in line with the valve stem to moniter if the rim is moving inside the tire, and with new slicks, the mark normally shows that the wheel did move a bit on the first few hits, but never much over an inch, and then they stay put.
Personally I don`t think your Mustang sits up all that high, rather I think the taller tires F&R makes it look higher than it actually is, suspension geometry wise. I don`t think your Mustang sits up any higher than most Super Stock Mustangs. (Other than maybe Richard Allens 68).
By the way, the 9x29 Hoosier slicks I use on the Mustang, are the D07 compound, which is designated as a "stickshift" application. They are not a true stiff sidewall, but they are stiffer than the regular "normal" compound (D05 I believe). If a similar semi stiff sidewall is available in the size you need, that could be another option. I have to think that the stiffer sidewall has to help with the tires ability to maintain air pressure so well.