These dimensions are for a 101" wheelbase chassis. That is to say a body with a 20" pickup bed would be the best choice for this chassis. If you are building a modified with a gas tank behind the body this frame should work just fine for you. If you want to use a turtle deck on the back, then add 4" to the length of the rear top rails for a 105" wheelbase. The 105" wheelbase is the longest you can build out of a 20' length of tubing. Some manufacturers offer a 14" box also. I prefer the 20" version because it gives you a little more room for a gas tank, battery, etc. inside the box.
You will find your build will go faster if you have the major components before you start the build. At a minimum having the body, pickup box, engine and transmission, rims and tires that are close to the height of the ones you plan on running. It makes it a lot easier to reference the location of future parts and assemblies. Then to, you can mock everything up, climb in and make some really disgusting noises. Just don't let the wife and kids catch you.
When you get around to setting up the engine in your chassis, think about using a dummy engine and trans. If you can come up with a block, heads, pan, water pump, dizzy, intake manifold and a trans housing, you'll be all set. It's sure a lot easier to move around in the chassis. With the dummy you can set up all of your mounts, build headers, check for clearance issues, cut your firewall, build the trans hump, just to name a few reasons. Look around. You can likely find most of the pieces for little or nothing.
The same goes for rims and tires. There is no reason to have your good set of wheels around during your build. A set of OEM rims and tires that are at least close to the height of the ones you are going to run will help keep the cost down during the build.
I've even gone so far as to build a mock up radiator. You can use 1x2's and cardboard or thin plywood for materials. I used pieces of an old hand rail for the inlet and outlets. A couple of sheet metal brackets for the shell and the frame mounts and your in business. Radiators get dinged up real easy. When you do buy yours, cut a couple of panels out of a box and tape them on the front and the back of your cooler. That will help keep those nicks and bent fins from showing up.
Speaking of cardboard, when it comes time to build your gas tank, cardboard is a lot more forgiving than metal and it’s cheaper too. With some searching if you can find a box that is close to the size of your battery, the battery tray can be built. A good source of posterboard for templates is as close as the nearest liquor store. Most of the signs they use are made of the thicker stuff. You can cut it with a band saw or a sabre saw. It even sands well with sandpaper.
Well, there you go. You can build your frame now. The front and rear suspensions are next. One last thing - if you are at all in doubt about your welding skills, find someone who you trust to weld up your frame and suspension. These are the backbone of your car and you don't want to compromise them. A little money spent here could make a big difference in enjoying your T.