You could simply add plugs and a momentary spring loaded push switch to activate the plugs.
Not rocket science, but it will do the job.
Simply hold the switch 'push on' for 15-20 seconds and start her up.
You are then the 'relay' LOL, it works!
I knew one chap thet simply held the wires between his fingers to start his engine, and he did that for years.
Just limit the heating for a short time, like count thousand one, thousand two etc and at 15 try her out. If no start repeat 'til she starts.
I ran my GMC 6.2 L for years that way!