if its the original 160mph mechanical driven speedometer its not a button , its a knob and needs to be spun to reset trip odometer.
the avc kit came with a black anodized shaft extender that was knurled at the end that extended past the dash bezel , the other end had a hole in it that slid over the metal sticking out of the speedometer glass and stayed put with a tiny recessed allen bolt
rough guess its 2.5" but id have to find and measure the one i have
on one i used a short piece of 3/16 brake tubing with the double flare end at the dash side (made for a smooth knob end) and a little epoxy in the other end to stick it to the speedometer shaft