The only thing it will make easier is with the clip you will get the plugs you need under the dash and the hood that have to be swapped so all of the dash and EFI harness will work. But as for those if you can find the part number for them I am sure you can get them from the dealer and the pins as well.

Another thing to consider is even if you get the clip or the engine with an uncut harness you will still have to cut it if you want to put the ECU in the stock location behing the glove box. The engines are coming from a RHD car and the harness will need about an extra 3 feet of wire or so to make it to the stock location of a LHD Supra. Your either going to end up with a splice area in your engine compartment or maybe you can find some sort of connecters at radio shack you would need two at least complete with pins. The problem with that idea is getting the plugs wet in the engine compartment so the splice area sounds better now that I think of it unless your going to use a potting compound to seal the plugs from moisture.

Damn. I thought I was the only person that still used radioshack.

I ran my harness through a new hole in the firewall under the speedo cable and behind the heater core and it reaches just fine in the stock location. But if you want to solder like 170 wires you go right ahead.