Use the toyota harness. They are fairly similar to GM wiring. Enough so that it can be done reasonably easily. Also most of the aftermarket harness' are VERY generic. No connectors, requires lots of splicing etc.
Assuming you are working on a '76 FJ40:
There will be one wire to the starter(the thick guage(black and white, i think) that needs to be hooked up to the start side of the chevy starter soleniod. I would recommend untaping the wiring harness and moving the thick black and yellow wire(that runs to the R fender to the coil) to exit the harness under the dash and come through the firewall near the original temp wire. Connect this to the IGN side of the HEI(or the + coil if using points). Put the toyota oil pressure sender near the distributor(to the left) where there is a plug on the rear of the block. Put the temp sender on the RH cylinder head in the plug above the starter. Now you can use a little creativity to get these wired, but you could try this: Use the existing wire for the original Oil pressure sender and connect it to the temp guage on the RH head. Connect the original temp sender wire(that comes through the firewall near the choke to the oil pressure sender by the distributor. Now, remove the instrument cluster and find the two wires for both the original oil pressure wire(yellow/black?)(now the temp sensor) and the wire for the original temp sender(now the oil pressure)(Yellow/green?). Cut both wires about two inches from the plug. Now solder the wires such that the yellow/green wire in the plug connects to the yellow/black wire in the harness. repeat for the yellow black wire from the plug and the yellow green in the harness. This will switch the senders, and allow the original temp wire to drive the oil pressure gauge and the original oil pressure wire to drive the temp gauge. I think that will work fine...Anyone else agree?
Sorry for the long reply, but hope that helps. If you have questions email me.