AHC & WI: Successful Spanish invasion of Portugal during 7YW

How can we make the Spanish invasion of Portugal successful during the Seven Years War? The original aim was for Porto, but could they go even further? How can they defeat the guerilla uprising? How much territory could Spain keep in the peace deal?
 
The only parts of Portugal that Spain is interested in are Olivença and the Sacramento Colony in Uruguay, because they are a continuous pain in the ass, being smacked in the middle of Spanish territory.
 
a bit of proper planning might have helped. they counted on living off the land, and when Portugal went scorched earth combined with them getting bogged down, the Spanish were caught without supplies. having more competent leadership would have helped, too.

I've often wondered what was to be gained if they did succeed. It wasn't going to win the war against Britain. I wonder how much Britain would return to Spain in exchange for something that wasn't theirs. At that point in the war, France was ready to call it quits, and OTL had to buy off Spain with Louisiana to convince Spain to call it quits.
 
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