No tbird or Cougar ever had PI heads from the factory. They only came out in 99 for the Mustang, and in 01 for everything else. There is a difference in the heads and intake setup from 94/95 to 96/97, which may be what you are thinking of. 94 and early 95 heads had a problem with wearing out the valve guides, which causes the engine to burn oil. Starting in late 95, and on all 96 and 97 cars, there is an updated version of the NPI heads that has the better valve guides, and that also allows you to run a larger cam without causing coil bind. Also for the 96 model year, they changed from the all aluminum intake manifold to the plastic one, which is of the same design as the 96+ Mustang. If you are going to do a PI head and/or intake swap, it makes it much easier to be starting with a 96+ car. As for the one car shaking, that sounds like a misfire problem, that may be as simple as needing plugs and wires, or it may be a cracked intake manifold leaking coolant. Either way, I would not let that sway you from picking the right car. There are plenty of more expensive things that can need repairing, like front suspension, rust, or transmission issues, so look at the whole picture, and maybe see if you can get the 96 cheaper due to the issues that it has.