^ LOL, yeah SRT and 3.7 use the NAG which is proven trans and rated at around 450ftlb of input torque (im not doing conversion lol) hence the 580NM in Wa580
I wouldn't worry about your trans if you had the NAG, but seems you have the RFE which Id tend to say isnt as proven or beasty as the NAG in the SRT (thats why it got it) it would just need some serious TCM tuning to allow if to change shift paramemters to accommodate for increase hp and all new motor parameters (tricky stuff, you'd need to call Dave at B&G or Andy at PWR or Bill at Paramount or even Eddy at SHR, someone like that whos in the trans game and know these platforms oh so well.) So try and grab that w/ the 6.1 if you can
As far as a motor, if you get anything beyond the 5.7/6.1 with all factory electronics you are going to struggle making all the modules and oem features of your truck work as well as risking many other electrical issues down the road.
That said what Marlin said is easiest way to go. Grab an aftermarket stand alone ECU and have someone familiar with Hemis (one of guys above maybe or even Westbury Jeep up in NY could help) set it up.
Main issues will simply be getting everything to mesh and work and then be reliable. If you go OEM you KNOW it will work as designed but dont forget youd need:
6.1 crate with ALL electrical wiring for motor (PCM/wiring)
Trans (will come w/ motor from most places fi you go used) (TCM module/wiring)
ESM/TCM/PCM reprogramming from dealer I believe once its all hooked up
Im not eve sure as the exact swap hasnt been done but when you try and swap another VIN part into diff ending letter VIN I dont think the modules like to recognize each other and canbus freaks out. Not exactly positive, but Im sure its a PITA getting electronics from the 6.1 part to work with 4.7 electronics, hence the huge headache!
Then as far as other stuff goes, to retain functional AWD, you'll need:
NV146 (SRT t-case) or aftermarket equivalent (Bill has something)
Front/Rear diffs (SRT8 has the Dana 44 3.73s) Not sure how long the factory open diffs in the 4.7 or the oem half-shafts will handle the 6.1 torque??? would suck to find out
So all in total this could be had for around $10k (+- 3k or so) Id say pending what kind of deals you find/get
Then you'll need all the labor/tuning which will add a wee bit, maybe another $3-$4k realistically so say $15k+ at best to get her up and running.
For that, Id sell/trade your whip and finance the diff for an SRT8 or something similar
Then you have the real deal