Renegade6;
The 58-64 car is a very unique piece because of the original unique frame design. true, You can get an X frame to handle and ride very well with one of their systems. I know that for a fact as I worked for RideTech for 13 years and they make good stuff. On a different note I am very familiar with the X frame cars, their nuances and their downfalls. What you need to determine is to what level of performance and drive-ability that your wanting to go to.
When GM desinged this platform of cars, it was actually pretty revolutionary. They were shooting for a universal platform that could be used across all the models of their full size cars very easily. The design also gave a lower floor for increased head room with a lower roof height.
As revolutionary in 58 as it was, the inherent design issues did arrise especially when attempting to build a world class performance car by todays standards. The major set back is the mount of flex that you get from the center section of chassis. There is simply not enough surface area over the body to dispurse that kind of load. That is why if you look back at early drag raceing photos of the early 409 cars, you can find some where one front tire looks to be about 2-3 feet in the air and the other three tires flat on the ground. That is because that rotational load is now being transfered through to the body to try to absorb the load.....Another reason they made great hydraulic cars too.... there goes those nice gaps

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The other issue that they have (especially when lowering the car to what we consider a performance height by todays standards) is the routing of the exhaust. Typically your coming off of the back side of the head, it dumps down to a collector that then has to be routed under the forward kick up to the outside of the rail. That usually ends up being the lowest point of the car... great for collecting man hole covers too.
By going to a full length perimeter chassis, you are building as rigid of modern platform as you will get by tying that body and perimater frame together. You will also be able to achieve ideal modern performance suspension geometry incorperated with a power rack and pinion. If you are also wanting to set it up with Ridetech Air Ride, we can do that as well. Our chassis are engineered to utilize their components with no compromises.
Here is an image that I just stole from a thread
https://lateral-g.net/forums/showthread.php4?t=36844 on our sponsors section where we posted a thread about these specific chassis.