If you use U-Channel then you can cut away the top and allow you to weld the entire length of the floor ( this creates a torque-box ) so that you have additional strength added to the car.
Set up the U-channel under the car and lift it up into position as best you can till it hits - then measure the biggest opening above the U-channel and use it as your guide measurement - this will give you the floor level - use the measurement every inch along the frame connector placing the mark that distance below the floor level on the connector and connect the dots - then cut it out - and you'll have it.
Square tubing will surely help, but the U-channel welded to the floor is by far the strongest way to do it.
( if you want me to draw out a picture to better explain the process - I'll be happy to draw it out for you . . . you'll have to do the same thing if you buy them - they are not perfect and they do need to be tweaked to get them installed in the car )
Here's a quick sketch of what I mean . . . hope this helps . . .
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