Actually there is a rear-drive transmission for the S series engine which has been around for ages; the W55 is available with a suitable bellhousing, and of course it's compatible with a W58. Here in Australia they're found in 83-84 Celicas and Coronas (with the 2S-C engine), but I'm not sure if they're common in the US.
Personally I'd put a 3S-GTE in a Celica, not a Supra; the extra weight of the longer chassis isn't necessary for a 4-cylinder engine. Over here I can pick up an SA63 Celica and the 3S will practically bolt in! You need custom manifolds though (to suit the RWD layout), so it's not a cheap option.
As for the JGTC Supra, I strongly doubt it weighs 1800kg when a MkIV weighs less than 1600kg from the factory! And what's this about a 3.4 litre Skyline? The GTR comes with a 2.6 litre engine (RB26DETT), and the biggest RB series engine is 3.0 (RB30E), so I don't see how they could have come up with 3.4 litres...