If you are going to modify or weld up a hot rod chassis from scratch, a big thing to consider is how you are going to stop the distortion of the chassis as you weld it up.
The other thing to consider is setting the ride height and wether you want to have a rake built into the stance of the hot rod. That is when the front sits lower than the back.
Either way the chances that your concrete floor of your garage, factory or workshop are not that level at all so the best thing to do is to build yourself a cheap chassis bench out of RHS square tube.
The chassis bench shown here is a good example and it can also double as a big flat surface welding table
You can down load the plans for making this chassis bench from here for free.