Don - KM4UDX
Tom -- many uBITXers have faced the same issue...digital modes...long duty cycle...heat...
My approach was to hacksaw larger heatsinks from an old Pentium computer motherboard's CPU. They have a lot of heatsink! I then drilled a new hole in about the right position. Screwed the 510 to them and immediately realized that I failed to pay attention in class. If you just screw'em together like I did, they can't touch anything...at all...even air might be a risk...just kidding... But, if they socially isolate, and ho hand holding, then you can cook'em up.
Better to do as other suggest: Use electrical isolation.
The good news here...is if you add some mondo head sink, and/or fan...then you can use a old laptop 19v (typical for dells, etc.) powersupply to drive the (red..or...brown?) wire on the little power connector to the mother board that drives the finals (only). That will feed the 510s with (typically) 19.5 volts, and you will (almost) double your juice out the antenna. If you have a old 24 volt power supply sitting around...welll...then you will definitely 2x your juice...and make your uBITX a real groovy hot rod. yeeehaaa.
Here are some pics of the salvaged heat sinks next to the originals, and a pic of them installed. Yes, they practice social isolation. hahah.
I eventually cut a hole behind the heatsinks and added a fan. I did that after my original IFR510s went all Chernobyl on me under...wait for it...too much heat...hahaha.
While I am clearly a slow learner, the mighty uBITX has stood up to the abuse.