the 68-70 Dodge or Plymouth B bodies are pretty straight forward, sure it takes a little wiggling, but it'll fit in there, in fact you'll find your self probably removing it a few times to make some tweaks, or hammer dolly some flanges, etc, etc, so you'll get the knack of it, if your doing a 68-70 Charger, than yeah it can be even more frustrating with the sail panel of the Charger as it's more difficult to slide into place, make sure once you get it into place, just tack it in position, make sure all trunk extensions fit properly, test all glass and trim, bumper fit, wheel opening moulding, wheel house,any trim, etc, etc, cat wisker trim, make sure if there's any tabs/flanges, etc that you need off the old qtr are transferred to the new qtr before/during install,...cause there's nothing worse than installing and welding up any qtr used/repro/NOS and find out it's off, shaped differently, or missing something that needed to be added before it went on,...and do invest in leading the roof and rocker seams as original, you'll be sorry later if you bondo them up, vinyl roof car or not...
mike