What is this thing and should I use it on my 69 gmc

[MYOUNG]

I am slowly reaching the finish line on the complete front end swap on my 69 GMC C2500

I bought a complete crossmember with the brakes and wheels still attached including the torch cut frame rails still bolted on. I tore it all down and rebuilt it and I am now going to bolt it into my 69 GMC frame but I noticed that the 84 frame had an internal clamping plate or brace of some kind that looks like it would also fit into the 69 frame. The 69 Never had this part but perhaps it was added in later years to make the frame stronger or prevent it from getting loose??

Do I need this thing or should I add it to my 69 frame??

Thanks