Decided to make this a WIP, whilst it works; so as the title says, "smallest N64 in world or bust" - I think that is apt! 
So, onto the WIP:
Three battery packs would be a bit on the heavy side, so decided to go for two. If using two packs, thought I would rip open one of the double height Canon BP-911 cells I used before on my Multi-Platform system; I have 4 cells, only need 2, so let's use one! The pics are blurry, but basically they are 7.4v and 4.4amps. Given that 8.8amps powered my Multi-System with the N64 for (as I recall) just under 7 hours continuously, this system should run for around 3.25 hours or thereabouts; which is perfectly fine.
Lift up the positive and negative pins after snapping off the corners of the casing with heavy pliers to access the strips. Use a screwdriver to pry up the casing halves.
Then pry up the shorter part.
Use clippers to weaken the casing and use pliers to bend back the plastic to break the cells free from the casing. Be careful not to damage the cells.
Result. Notice, the protection circuitry is double height.
For purpose of illustration, I put a 4mm tall VGA memory heatsink on one of the RAM chips; the main processor chips are a bit taller of course, by a couple of millimeters. As you see, when the expansion pack is trimmed down (by about 2-3mm) the top of the batteries will be level - this is why there is no point relocating the expansion pack - no advantage, the batteries have to be somewhere of course!
As daftmike said before, it is hard to do a WIP on the wiring as you can either use a multimeter or you can't; however I will give pointers - use kynar wire, it is thin, single strand and strong - IDE cable often breaks easily and isn't normally obvious that there is a problem. I am keeping the wiring fairly short, approx 3.3 inches long, although some wires will be a bit shorter and some maybe a bit longer - trying to keep them about the same, however in order for the game cart to be visible with the game graphics showing, the cart had to be inverted.
Its taking a long time to wire, much longer than anticipated, as the soldering is really fine. After each wire is connected, it has to be checked there are no shorts too.
The yellow wires are all grounds, going to wire those up last. To make the wiring a bit tidier, I use thin pliers to bend some of the pins on the cart slot together if they shared common connections.
Continuing to solder more wires, back later! It is very tiring on the eyes to do such fine soldering work, so this will take longer than originally thought - need to take frequent breaks or I go a bit "boggle eyed".
Decided to test board slicing with another board first, when I am ready for that stage, as it would be depressing to destroy the board after getting the cart relocation working!
UPDATE: - Boy, this is mentally really tiring, having to concentrate on such tiny contacts. Anyway, got a few of the wires done. They are all right next to each other - if you look at the chips on an N64 board, you see how small the areas are!
I have a system, try and wire adjacent wires to the corresponding pins on the cart slot, rather than the other way around; otherwise it would be amazingly hard to solder a wire between two existing wire connections. I put a black dot on the outer side of the cart slot to indicate they are connected.
UPDATE 2: 50% done. Had minor scream when a tiny blob of solder attached one pin I was joining to a neighboring pin, towards the top of the pin itself. Fortunately, managed to resolve with some thin desoldering braid. It was a bit touch and go though! This fine soldering is fairly straightforward, but not for the fainthearted, that's for sure. Doing well though with my £4 cheapie pencil tipped 30w iron!
UPDATE 3: 100% done. Only need to wire the ground to the board. Couldn't work out where pin 46 went, looked at daftmike's pinout and saw that pin doesn't go anywhere! Nice to have confirmation!!
Looks messy. Hoping to get wires shorter if I can, but first things first. Tomorrow I will check my connections, then cross fingers and test to see if all works or not. Project will either then be dead before it starts or continue to the next stage!
Hoping the kynar wire proves suitable for such work - mind you, daftmike is using IDE cable, so I should be hopefully ok.
Fingers crossed for tomorrow!
