That one you linked to is an entire reproduction cab that's been stretched. I believe it's fiberglass,
here's their full website. They also reshaped the rear fenders so they look more like the front. I don't like it, but custom stuff is very much personal taste.
Some guys stretch it so the door is the same length but you just have extra cab behind the doors. Too much hassle for not much gain for me. I fit just great in mine at 6'2", and it's very comfortable on long trips. Part of the fun and allure of these old pickups to me is that the cabs were much smaller and just the way everything was shaped and fit together. Upsizing it to more modern dimensions takes away a lot of the unique characteristics of these old trucks, to me.
Whatever you do I hope it turns out as you want it.
