First:
I'll assume you have a roll bar. Othewise, you'll be wishing for just a lap belt if the car ever rolls over.
And, be sure the shoulder harnesses are secured to a point that angles no more than 15 degrees down from your shoulder. For example, there is a "hump" in your car directly over the rear axle (it seperates the trunk from the passenger area). Bolting the harness mount to the highest part of that "hump" will still make it too low, unless you are fairly short or angle the seat back very far.
My car has a harness bar that bolts accross the tops of the rear wheel wells, but I do not have a convertible top to worry about, and my seat is less than 1" thick. Welding a brace to the roll bar would probably be best for a street car.
I often work tech inspection when our club has "Track Days" (for street cars) and I have a lot of kids that come with bling-bling Hondas, etc that have improperly mounted harnesses. If their car still has the stock three point, we make them use those. If not, we send them home.
By the way, our club does not actually allow four point belts even in they are mounted properly. We require five point to prevent submarining under the belts. Stock, three point belts tend to reduce submarining due to their inherent design.
Honestly, I have pretty mixed feeling about even a three point seat belt in an open car. When I bought my new Miata, I went home and ordered a roll bar the same day. I'm sure lots of folks have a different view on this point....but I won't drive an open car on the street without a roll bar.