You have little choice given you lot dimensions. Even at just 70', one should have a 40' turning radius at the entrance and the same after the conveyor to ensure proper turning. Skinnying these ditances down to 35' at each end instead of 40' will mean that for some long wheel base vehicles, they will have to pull up, back up and pull up again to gt in line with your conveyor guide rails. However, the vast majority of your customers will be able to make it with 35' (provided it is a left hand turn and not a right hand).
Remember too that if you decide to do ANY vehicle prepping at all, for whatever the reason, it will have to take place on the entrance pad before guiding the vehicle onto the conveyor.
If you make the conveyor any shorter, it will substantially and negatively impact on your washing, rinsing and vehicle drying as well. 70' is absolute minimum 90'-100' is the industry average.