This may sound like overkill, but there is no such thing when dealing with a 30 year old electrical system. LEDs are fine, but your headlight should not dim due to the brake light very much if everything is working well. Fix the problem, don't just cover it up by drawing less electricity.
Here's what I did and I would suggest for you. Check all grounds to make sure they are secure. Also, take apart each and every wiring connector and use a Q-tip to clean the female connectors (mineral spirits or WD or something) and clean all the male connectors the same way. When you unplug and plug in each connector, make sure they are snapping together securely. If they are not, use a pair of pliers to slightly squeeze the female connector so that everything is tight when it goes back together. remember that there are many connectors inside the headlight bucket, so make sure and take care of all of those. This is probably the most important point- take one connection apart at a time, clean it and put it back together. Also, clean all connections at the fuse box and check that they (including the fuses) are tight. Remove all lights and clean the sockets.
When I did all this, no other changes, my indicator and signal lights were noticeably brighter. I was running a 55w/60w headlight and I never saw the light dim with the brake lights while riding. Not saying there was not some dimming, just nothing I could notice.
Simple but somewhat tedious tasks like this are what lead to bikes being parked for years at a time. It's not all that fun, but methodical, regular maintenance is necessary. After you are gotten the whole system working like new, if you want LEDs, go for it.