well that's a different question then
,
from what I have seen the best deal money wise is the Gotts mod, its cheap HP, most people pay less than $20 for everything. After that the Gryphon is the next best option. you wont get gobs of hp with it, but it sure makes the truck run better. I had a 2001 with 135000 miles and i put a regular gryphon on it and I noticed a big difference. so yes with that many miles it is worth it.
. The other mods are nice, but I cant say which is better than the others. the UDP's and Efans seem to have about the same performance gains, but the fans are usually more expensive. The exhaust, if done right, will do a decent job of adding power and economy, but if done right it is by far the most expensive.
That help this time?