I have three FT-1000D radios, which are really in the same ballpark as far as Yaesu vintage radios go. Each has been modified to be able to transmit on 60 meters. I have been operating the one I currently have in my station on 60 meters quite a lot lately, and not had any problem. Well, the thing might blow up tomorrow... but I don't think so.
That having been said, my FT-1000 radios were designed for 200+ watts output, and might have a bit heftier LPF section than your radio (not sure on that).
I'm not trying to tell you to do ANYTHING, just my experience.. but the alternative would be to dump the 990 and procure something else. I can tell you, I'm not interested in ANY alternatives (in fact, I'd love to have a 990). But, like I said, I have three of mine.
Thank you for your comments.
I will never sell the FT-990. It is tough as nails and it has been the best radio at field day for the past 3 years. Everyone wants to use the old 990 after a few hours of operation on newer rigs and seeing the difference in how well it is working.
If I have to, I will put it on a shelf while doing 60 meter opps. It will be my main rig for a long time to come. I have had several newer Kenwoods and two newer Icom rigs and I always go back to the 990.