[Dwight]

you can buy a tail pipe expander and adjust the diameter of the pipes till they are tighter. I had to do that on my GT 40 bundle of snake pipes.

I have installed flanges on a few slip type side pipes. It's not that hard if you can weld.
I like to use 1/4" plate (5/16" is better) about 6" x 6". Use a hole saw to cut the holes for the pipes. Drill four to six holes for clamping the two plates together. One in each corner and maybe a couple in the middle. The thicker plate requires fewer bolts to clamp together.
Remove the headers and side pipes from the car. Bolt them together and pick a place where you can cut both the header pipe and side pipe into. Remember to check to make sure your new plate will not interfere with anything.
A Port-A-Band is a great tool for this part. Cutting through all four tube (eight pipes) you will remove 1/2" of pipe length for the two flange plates.

Clamp the flange plates in place and tack weld to the pipes. Remove and finish your welds inside and out.
I buy laminated aluminum sheet from McMaster Carr to make gaskets. They work better for me especially if the plates are warped.