This depends on several things....The money that you're willing to spend and what kind of daily driver that you're used to driving.
Higher performance engines make their power at a higher rpm, so a "stall converter" slips more at low engine speeds. This allows the engine to more easily rev past the low rpms and get into the power range without the converter fully engaging. Think of it like when you rev up a manual transmission car before releasing the clutch. A low stall converter would be like releasing the clutch at 1500 rpms. A higher stall is like releasing it at 3000 rpms.
Overdrive, whether in an automatic or a manual transmission is THE way to go when you want to have flexibility in a street driven car.
A torquey Big Block will do fine with a 2.76 or 2.94 axle gear once it is moving but it will not feel as snappy from a stoplight as it would with a 3.23 or 3.55. Sadly, this is an area full of compromises.
The TKO series has been replaced by the TKX. Even the TKO requires some floorpan modifications but nothing structural. The TKX is slightly smaller but you're still looking at cutting a hole for the shifter. These Tremec trasnsmissions have shifters that must pass through the floor just like any other manual transmission. Stock 4 speed models had the shifter and linkage on the side. The Tremecs require a hole on the top of the transmission tunnel. It is not difficult to do.
Actually, the TKO had two overdrive ratios available, .64 and .82. I have the .64 ratio. It is excellent. The TKX supposedly only has the .64 ratio.
What you'll find with the 5 speed is that no matter the condition, you always have the right gear to use. Mine has the 2.87 first gear ratio. This is deeper than any regular Mopar 4 speed so it gets the car moving a bit better. The .64 overdrive means that at freeway speeds, the engine will now rev 36% slower than a 727 or a regular 4 speed. Freeway speeds of 3000 rpms will rattle your brain after awhile. Most if not all of us have become accustomed to lower rpm freeway operation with our later model cars. The interior noise is reduced, the engine noise is reduced, the car gets better mileage and the road trips are much more comfortable. The Tremec 5 speed swap was one of the best things that I did to my car. I went from maybe
10-11 mpg to 15 in my 440/493 Charger with a 3.55 axle gear.
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While that may not seem extreme, any increase in mileage means an extension of time and miles between fill-ups at gas stations. If you've ever worried about making it to the next station, longer range really eases your nerves.
This thread is long but it covers a LOT:
Tremec 5 speed conversion in a 1970 Charger
The 6.4 HEMI is a great engine but will cost you about double to swap in compared to a 440 when you factor in the fuel system, electronics, special mounts and exhaust system. The parts to do a 440 swap are easier to find and a lot cheaper. A stock 6.4 will make about the same as a mild 440 but get better mileage. The decision as to which engine to go with is up to you. Both have pros and cons.