Just wondering how big of a pain it is to replace the front panel on a 73-79 long bed. How much bracing do I need before I cut out the panel and where is the best spot to brace the bed?

 

It is not really that big of a deal in my opinion. I set my bed up on top of some 55 gallon drums and worked on it that way. I did not do any bracing. I did measure from corner to corner first, so that I could adjust the bed if needed to remove a diamond.
Just take your time, andyou will be fine. Make sure to buy some paintable seam sealer to finish out the edges when done.