Copper rtv. That's what I did. Works great and no more leaks.
99 Silverado 4.8l : Soon to be 80E swapped, then 6.0 swapped, ......
05 Yukon Denali 6.0l : Nelson Performance Tune, ......
8X S10 Blazer on a 8X 3/4 ton frame : 5.3l swapped, Nelson Performance tune, Schoenfeld LT's, 226/226 .598/.598 112, TH350, planetary doubler, NP 205, locker, spool, 38's, ......
It isn't gonna come out too easy because those bolts have yellow locktite on from the factory so i doubt penetrating oil will do much but welding the nut on might be a good shot because it gives you something to turn it out with and may heat up the locktite enough to break it free.