Neutral safety switch

Reverse lamp switch moved from inside to outside in middle of 1968. I'll assume you have reverse lamp switch on the shifter but... You didn't say what floor shifter you installed. If the OEM floor shifter linkage isn't all the right pieces and/or adjusted just right the opening in the console wont let the shifter go into park far enough to get start. Get that adjustment correct and shifter may hit console before you get fully into low. I surely didn't want to miss park and have a roll away so I forfeited some of how well I get low to ensure park paw engagement. I never use low anyway. I've read that there is a slight difference in length of the first little linkage levers on the trans.