AT vs ST chassis differences? Options

[Nespeed]

Just looking for info on the major to minor differences between a AT and ST chassis.

[Special_Edy]

AT = A series engine
ST = S series engine

The GT(ST204) and ST(AT200) are an identical chassis, the ST(AT200) has different engine/transmission mounts that are designed for the A series engine, whereas the GT(ST204) SSII/SSIII(ST202) and GT4(ST205) are all equipped to accept S series engines(3sfe, 3sge, 3sgte and 5sfe)

The GT4(ST205) has a different floorpan than all the other models of celicas, from the firewall back, so that it can fit the rear differential and driveshaft. It should also be noted that the GT4 has an E series transmission instead of an S series, so the mounts are slightly different.

But to answer your question more directly everything from the GT4 front clip will bolt directly into your AT200, with the exception of the rear differential(if you want AWD) and the Passengerside engine mount(not sure but Ive heard AT200 owners have to drill out some premarked holes)
You will need either an S54 transmission from a GT(ST204) or an E153 from a camry, mr2 turbo, or solara to name a few. The AT200 transmission will not bolt directly to the 3sgte motor. The E153 is the transmission you want behind a 3sgte though, the S54 has weaker internals.