I just bought this car and I am doing a frame off restore and I have heard that that brace gets in the way of the exhaust.

As I made my way disasembling the underside (cutting the exhaust up) I was looking for this thing but there isn't one there. This car was all original so it seems that it never had one.

Did all converts absolutely have one?

If I put in frame connectors do I need to put one back on?

I am planning on 400 + horsepower and fatty tires w/4 wheel discs,,,,so that will probably put a little extra stress on the car. This my first convertible ever so I am a newbie in this area.

All Camaro verts came with the x-brace. It attaches to 6 welded in nuts in the H-brace under the floor. If you look, you'll find the 3 threaded holes on each side of the tunnel, the first one is a few inches back frome the crossmember.
The part is reproduced, and recommended to have, SFM's or not.
Many "commercial" exhaust systems may interfere with the brace, usually this is because the H-Pipe will hit the driveshaft before the system is high enough to clear. Some have been able to shim the brace for the needed clearance. Some have had thier commercial systems professionally installed above the brace successfully. Some have fabricated replacement braces that fit over the exhaust. Others have had custum systems bent up. It's not hard for a muffler shop to bend up a 2.5" system that will fit right, not sure about 3".