Which exhaust system from borla would fit on an 84 z-28 (currently 305, but will be soon upgrading to a 350)

 

The only one designed for the 84-92 F-bodies.

From Jegs:
part number: 157-14888
*Camaro/Firebird 1984-92, 3'' Adjustable, Side Exit, Dual Round Angle-Cut Tips

More info:
The Adjustable System is a unique, dual-purpose system which starts at the rear of the catalytic converter and goes over the rear axle. From there, the exhaust flows into a Y-pipe with exhaust tips on one end and a high-flow muffler/exhaust tips assembly on the other end. Four interchangeable plates come with the system which allow you to adjust noise and performance levels. Using the solid plate diverts all of the exhaust through the muffler (street legal). The other plates have holes in varying diameters, or you can refrain from using any plate at all, running a straight-through exhaust, which can be easily converted back to street-legal in a matter of minutes. Clamps and necessary mounting hardware are included.

*Note: Four interchangable plates allow you to adjust performance and noise levels


Hope that's what your looking for.