ORIGINAL: Uncle Vinnie
Since you have the G3.0 motor, I wouldn't upgrade the motor, get the 13T bell, with a exhaust for bottom end, and you will have enough torque to keep you happy.
By going with a the Os engine above you will be disappointed with the results, it will not be much better than what you will have.
Understand that Vinnie has spent countless hours supertuning his T-15 and has gotten better results out of that engine than any other poster in the MT2 thread, by far (I have read the entire 800+ pages, as well as his MT2 Jatokiller project thread). There would be a definite improvement over both the 18ss or the T-15 for 99% of users, and if history is any indication, the g3.0 as well. HPI makes a great big block with their 4.6 engine but has a poor reputation when it comes to small blocks, sorry to say.