Here's some dimensions of the topper:
1. height of topper from bed rails to cab height: 23 1/2" (sits above my regular cab by about 3-4")
2. Over all length of topper: 74" (front to back)
3. Over all width of topper: 59 3/4" (from outside to outside)
4. Width of bottom of ambulance doors (fits between rear bed pillars, just between rubber nipples for tailgate stop, flush with rear of pillar when tailgate removed): 53 1/2"
5. From bottom of ambulance doors to bed rail height of topper: 17 3/4" ( there is a small corner gap exposed where the bed pillar curves back into the bed side of my 2nd gen tacoma and the topper makes a right angle when on my truck.
I bought the topper from a neighbor with a 1st gen tacoma access cab with long bed. I think the 1st gens are slightly more narrow between bed sides. When topper is mounted on my 2nd gen there is about 1" of bed rail top exposed outside of the topper on both sides. I used four u-bolts around my stock tacoma adjustable tie down rings that are fixed to the side rails to fasten down the topper to my bed.