1-ton specific parts are not as easy to find as 1/2-ton parts and often when they are the same as 3/4-ton parts they still are not listed in catalogs.
My own thoughts regarding modifying a 1-ton frame versus finding a 1/2-ton frame, versus maybe something like an S-10 frame swap would center around how you really intend to use the truck and what engine transmission combo you plan to use.
In short, if you are going to stay Stovebolt six and straight axle front, I'd probably stay with the 1-ton frame and modify the length. Once you want to modify the suspension the 1-ton is different enough you will not be able to buy aftermarket pieces that simply work, they will require modification.
If you want to go Mustang II starting with a 1/2-ton frame will be much easier. If you want to go V-8 its much easier with the Mustang or other front suspension kit.
I have several projects and the one I'm most likely to work on any given day is a 1-ton. I'm modifying the stock frame with later 3/4-ton IFS and a Cadillac 500 V-8. My goal is to pull a loaded trailer fast and safe and all my frame, suspension, and drivetrain choices have been made in light of that goal.
One further thought is make sure you understand local laws. I've heard a number of Canadians say no welding on the frame. Where I live in Washington frame welding requires an engineer and a certified welder. Now I'm not going to pretend everyone obeys those laws, but you need to understand local rules and how you are going to fit your project into the picture. That might change your thoughts on modifying a frames...
