Six speed won't hold up under the torque. Hell, they barely hold up now with those that are racing them. More the clutch than the tranny, but drive line all the same...read thru some of the threads. We will have a few of them at the get together this fall at the Dragon in TN...might be worth the trip to join us, and check them out, get all your questions answered.
As for beefing it up, rearend is a start. There isn't a whole lot of beefing up, as the 55 motor will drop right in. Not up on the hemi, but there is one out east that was posted on here not long ago. Rudy is the guy to ask questions of. RCompart. He does the conversions.