There are two types of rollbar, thin/light cosmetic ones that will provide chassis rigidity and SOME rollover protection, and heavy/thick ones that are intrended for crash protection. The ones that Shelby made on the 67/68s were of the first variety. They are not recognized by NHRA or SFI, but they are better than nothing in a crash/rollover.
I have the Autopower bar (firt type) welded into my vert, and have researched the use of shoulder harnesses, I prefer the retractor but a "Y" is the easiest and safest. Most rollbar mountings for safety belts will connect to a crossbar not to the top. This arrangement prevents "submarining" and is the only way to meet track safety requirements, I do not know about DOT ones. Without a retractor you will not be able to reach the headlights or turn the key, even the shifter might be an issue. But the retractor ones (OEM Shelby style) do not have the excellent modern-style inertial locks, so their safety is MUCH less.
I am very interested in seeing what you guys come up with, I still want shoulder belts of some kind.