If you plan to be in a situation where you actually need a 5 point harness, you need more framework than a 1977 unitized body has to offer.
And the sheetmetal floor, is not a good attachement point.
Now, with proper framework and a rollcage, the 5 point harness can be properly installed without rendering the back seat useless. (albeit the back seat of a '77 firebird, was pretty much useless anyway, unless yer both small people,
)
Thanx
Shorty