[Number 3]

T-tops are still a huge hurdle. Today, only convertibles are exempt from the roof strength testing.

Here is the verbiage in MVSS 216:

FMVSS 216, Roof Crush Resistance, establishes strength requirements for the passenger compartment roof of passenger cars, multipurpose passenger vehicles, trucks and buses with a GVWR of 2722 kilograms or less. The purpose of the standard is to reduce deaths and injuries due to the crushing of the roof into the passenger compartment in rollover accidents. The standard does not apply to school buses and passenger cars that conform to the dynamic rollover test requirements of FMVSS 208, Occupant Crash Protection, S5.3 by means that require no action by passenger car occupants. It also does not apply to convertibles, except for optional compliance with the standard as an alternative to the rollover test requirements in S5.3 of FMVSS 208.

A targa top, like the Corvette is considered a convertible top. T-tops are not exempted from 216 and simply makes it tougher to pass.

Back in the day, roof strength was up to 5,000 pounds or 1.5 times the vehicle weight whichever was the least, if I remember correctly. Now it is 3 times the weight of the vehicle which makes it much tougher to pass. The NPRM was originally 2.5x but the final rule now in effect is 3x. So a T-top in old days only had to meet 5,000 pounds. Now......12,000 pounds for a 4,000 pound Camaro.