Kyosho flat rear wishbone available:
UM566 (RB5 Worlds)
UM521-1 (RB5 SP2?)
LA272 (Lazer ZX5 SP2)

You need 2 packs as stated as it is 1 front and 1 rear arm per pack. If you run the Lazer as a 4wd get 2 x LA272 as you can use the front arms on that.
Otherwise I'd suggest going for UM521 (not UM521-1 as that only has the 1 shock mounting hole on the non-loop side) as this gives you a pair of RB5 front wishbones which I have used on the RB6 in combination with the relevant front hinge pin mount (sorry can't remember the number as so long since I bought one), this can calm some of the agression out of the RB6 which is helpful in some situations.

Mods to fit: yes, you either have to use the RB5 outer hinge pins along with the little M2.6 screws to capture them OR you can do what most do and drill / ream the smaller hole out to 3mm and use the RB6 hunge pins with 2mm of plastic washers either side between the wishbone edge and the nuts.
Additionally you will need to use 2x1mm spacers for wheelbase adjustment (come with the arms) and obviously remove the 2mm spacer from in front of the wishbone on the inner hinge pin.

Mounting them: choices as can go on either way around BUT most people put them on so the loop is rear facing as this gives the best wheelbase range (the other way is 3.5mm different). If you do this then need to mount the shock eyelet OUTSIDE the loop with another eyelet ball / spacers inside the loop, this will need a longer screw of course.

The EXTRA hole : a lot of people drill another hole 4mm inside the existing inner one, this gives a more upright shock that really helps the car on our high grip tracks. this extra hole is "just" inside the loop, you may need to choose you spacers for inside the loop carefully or reduce their size a little.
If you do drill this EXTRA hole then please make sure that you put a screw through the original outer hole in the wishbone along with another eyelet ball/spacer to strength the wishbone as drilling the extra hole does stress the part a little higher.

Shock eyelets : as a rule of thumb use the kit "LONG" eyelets with the kit arms, "Medium" eyelets when using the 2 standard holes in the straight wishbones and "short" eyelets when using the drilled EXTRA hole.

With the big bores try either kit pistons / 400cst oil / red spring on the inside hole or if using the EXTRA hole a Losi Pink spring with 350cst oil is a good place to start. Even better try a 4x1.3 piston with 50cst lower oil grade.
With small bores I believe most run the 3B piston and similar oils to stated above with either a dark yellow or yellow spring respectively.

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