transmission options and questions

Hey all.
I will be doing a 5.7 swap into a 70 roadrunner and have some questions and need opinions or insight....

I am about to buy an engine and have it narrowed down to 2.
I can buy an entire wrecked 04 2 wheel drive crew cab Ram for $3500.
Or I can get an engine, tranny and wire harness from an 09 charger for about $2500.
both of these engines I will be ale to hear run. and if I get the truck I could part it to recoupe some cash...

My questions are what is the torque rating of the 545rfe and is it able to be built to hold up if I mod my engine down the road?

OR- would the nag1 be a better option but is it still really expensive to swap in and control?

OR- is there another better option? I'm trying to keep the cost down as much as possible.

So everyone knows. My plan is to get everything swapped in and going, to get all of the kinks worked out and then get another 5.7 and either do a stroker build or put a little boost on it. So this is why I have questions on the tranny because I would really like to only modify the tranny tunnel once..

what would a good budget 5 or 6 speed be to go with besides the tr6060 o t56? anything that would hold up to my plans?

I appreciate the input very much, thanks.