Find something that can be told electronically to clamp onto the throttle linkage rod and hold it in place which also doesn't release until told to do so. When you're cruising at the desired speed, lock it in place with a toggle switch identified as NO2 or something

. Perhaps a switch on the brake and clutch pedals can be used to release it - that way it will release "naturally".
If you're still more technical, you could use an electronic speedo sensor to send a signal to a circuit board, which you have made. Using a servo to control the throttle, it could be programmed to hold that speed. The cruise control end of the electronic speedo sensor might be used with a custom cable to drive the original speedometer, if so equipped. Use the same switches to turn it off.
Lawrenceville, GA