There are several ways to do what you want to do .
1 use 2x2 or 2x3 tubing and cut and split the floor pan to fit it into , weld a plate on the end of the tubing and drill holes in it to bolt it into the oem spring box , be sure to add bracing at the front cross member it is thin , but with your plan for the 500 inch motor you might need more tire and you would end up cutting all this out and doing it another way .
2 had a friend that was running a 68 roadrunner with a warmed over 440 . The car started to act funny when leaving the line , hook hard then started to bounce and move around , he brought it to me and wanted to go bigger with the tire size and mini tub it , when I pulled the carpet back I found that the floor had split around both rear frames where they were welded to the floor . It was an accident waiting to happen . Used 2x3 tubing split the floor and rear frame rails and welded it in to both . Just a reminder the tubing sticking up into the floor and the carpet will not fit as before.
3 Using the US cartool stuff is the best way to go , I would use some of their other frame stiffing items also , a 500 motor will produce a LOT of torque .. again just my thoughts .
The stiffer the better , have built a lot of cars for my customers over the years , built one for myself that was so stiff that you could jack up the rear on one side of the frame and it would pick up 3 of the 4 tires off the ground . Hope this helps John