A boat is a boat. The seventies were a time of totally over engeneered boats.
There is a good chance this boat would take the rig change no problems but you would need to strip it and check it well. Certainly put new stainless chainplates on it.
You might strengthen around these areas internally with glass and resin.
Check the keels for wear and get a low down from the owner of how he has used it. How many people has it carried at one time etc.
Two robust men giving that boat a hard time on the race course i=s as tough on the rig as you will ever be but beaching it etc is a different story.
I would be most interested in the rigity of the fibreglass.
It is also worth taking a socket set and lifting the beams. This usually tells the story.