You're not going to get an accurate speed by just running the cable to the Chevy tranny... Find a shop (local or online) which specializes in speedometers (yes they do exist) and get an adapter made... It's a little gear box which you run a short cable from your tranny into and out of it you run the Toyota tranny's cable. They should have the ratios for each, if not the RPM for Toyota speedos is the same for all Toyotas of this vintage and then just get them the end gear ratio that you're running and the tooth count off the driven gear.
Good luck and have fun!
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