TDR posts are 50/50, either pan or cooling line. I'd do some more reading and look for feeback from people who have added HP mods like you have.
-If that pan you have is Mag Hytec there should be a 1/8" NPT port for a temp probe.
-Diesel Manor sells a cooler line for 3rd gens with built-in port, not sure if they or someone else has one for a 98. 5.
-Geno's sells a sender bracket that clamps onto the outside of the cooling line. Probe never touches fluid so no chance for leaks. Seems to get good reviews but there has to be some heat transfer (indication) loss with this kind of set-up.
-I'm a solid Westach fan. They don't look as much like OEM but are very good gages. I bought the Westach with dipstick sensor. Very easy install. I plan on buying the Mag Hytec DD pan for my 06 and will move my probe to the pan port.