Any time you buy an aftermarket part it is on you to order the part that will fit your car as it is currently configured. If you have changed your suspension ditching the stock Saginaw recirculating ball steering gear with Pitman Arm and Idler for Rack and Pinion steering then do not expect any other parts you buy to fit your car as those parts are built to fit a stock as sold by Chevy to fit.
The same applies to headers if your heads have raised exhaust ports, or even use the Vortec style "D" ports to fit either (which are raised a tenth of an inch, but the attachment bolt holes stayed in the stock location).
I frequently read about people's complaints about getting a 700R4 to fit with their stock cross-member, it isn't going to happen.
As soon as you modify your car (such as bagging it and lowering so the frame rubs the ground) then you have entered the world of hot rodding where you alone are responsible for the car's safety and it's functionality.
What most do not realize is the design criteria on these older cars. The engineers built them to last seven years (the time dictated by congress under federal law for capital goods to last) and to comply with the road conditions and driving standards of the day (n the early sixties that was unlimited cheap leaded gas, speeds limit of 45 mph, and no safety or emission standards (except for the road draft tube that was replaced with a PCV valve in 1963).
So if you drop in a big powerful V8 motor in your car with a lot of go, then it is also up to you to upgrade the suspension (springs and shocks) as well as install bigger brakes to provide a matching whoa! A lot of people think of the big motor first (often replacing an old tired six cylinder stove bolt) then applying fancy paint before replace their interior next. Frequently they don't get around to the suspension and brake upgrades until after they wreck their car.