I have a 68 convertible with no "X" brace under it. The way the exhaust is routed it looks like it will have to be replaced to install the brace. I have never driven it with a brace under it and don't have any idea how much difference it makes. Am I taking a big chance driving it without the brace or have other people gotten away with never installing one. I was also thinking about putting subframe connectors under it. I would rather do one or the other. Am I better off replacing the exhaust and putting the "X" brace under it or are subframe connectors good enough. Any information is appreciated.

If it's one or the other, I would pick the connectors. If you need to replace you exhaust anyway, and like the stock look then go with the "X" brace. I went with both the "X" brace and subframe connectors.

My car has both. The X brace is bolted on with no spacers, and my exhaust system was custom bent to fit above the X brace.

If you go with subframe connectors, make sure they are rigidly attached with at least two bolts on each end, and replace the body bushings before installing them.

What are the "subframe connectors"?

There are two types, bolt on and weld on. They stiffen your car by linking the rear sub-frame to the front sub-frame. I believe the weld on will be more rigid than the bolt on.
Here is a picture of the bolt on type. They are available thru jegs or summit:

Thanks for the explanation.

Mine look like those in Joe's photo, just beefier, and fully welded to the floorpan brace and the subframes. Here's how they look installed:
Subframe connectors

With the connectors the x brace probably isn't necessary unless you like a stock look. The 69 convertible that I'm currently working on had neither the x brace or the subframe connectors. After finding all the rust in the floor pans and brackets, I'm surprised that it doesn't sag. My dad drove it for 10 years occasionally in the summers and we never knew that it was supposed to have the x brace.