Well, given that you now know the correct Bigsby variant(s) for your '98 SF IV which Hans provided for you, you have a choice to make... Do you want to mod the guitar or not? It's hard to say if you'd affect the value much, but these days anything from Westerly is considered a vintage (and highly desirable) Guild. However, as you say you've owned it a long time, you obviously want to keep it. So, my advice is install that Bigsby and have at it.
That's just my 2 centavos, and welcome aboard! BTW, that's a very nice guitar!!!
That's just my 2 centavos, and welcome aboard! BTW, that's a very nice guitar!!!