Let me be the first to say that anything is possible if you are willing to do the work. That said, I believe you would find that the NV3500 to be very hard to put into an E body due to its physical size. I believe that you would have to severely modify the firewall and trans tunnel at a minimum. Then there is the issue of modifying the T bar support cross member which would where the trans mount itself would need to attach. I think in the end you would not like the end result, the shifter would be pretty high I think.
Converting to a manual (4, 5. or 6 speed) isnt going to be a real inexpensive effort no matter which way you go, but this depends on what you feel is expensive and what you have available to you right now. If you have nothing, then you need to look at some of the kits on the market that provide pretty much everything you need. Sure you can do it the "Johnny Cash" way and get the stuff "one piece at a time" and perhaps get it together cheaper. I'd start looking for a A833, look at Craigslist you might be able to score one for a decent price. Keep in mind there are many versions of this trans and the most available is the Aluminum case with OD. Its available bc not many want them. You also need a trans that has the rear E body shifter mount.
Once you have the trans, you can then source the other stuff like the bell housing, flywheel (you will need an externally balanced version for a 360), pressure plate, clutch, throw out bearing, shifter and linkage, some method of clutch linkage (hydraulic or mechanical), 4 speed hump, you might need a new trans mount and drive shaft.
You live in a good time in that there are options, you could maybe find a Tremec out of something else and make it work but whatever you end up with will require some engineering and modification to make work.
