I managed to get a set of RXV Shivers into Australia to do this conversion.
I've stripped these apart and stripped the F8 forks too.
Here's what I've found so far -
The springs from my F8 are 0.5kg/mm and the springs from the RXV are also 0.5s.
Both are the same free length of 474mm.
The RXV spring seat is 21mm higher than the F8 but the spring sits 4.5mm further into the top cap of the RXV, so already there will be a preload height / static sag gain of 15.5mm.
The F8 used a spring preload spacer of 30mm.
The F8 & RXV cartridge bottom out at exactly the same length measure from the top cap but, as mentioned by a couple of the guys previously (and an exceptional write up too), the RXV tops out 40mm higher.
Top is the RXV shiver 45 fork, spring, stripped cartridge & complete cartridge. Then the F8 cartridge, spring and fork.
My plan is to use a 30mm top out spacer and gain the extra 10mm of travel - I'm 6'2" so touching the ground isn't an issue. I'll probably reuse the 0.5 springs and the stock 30mm preload spacer at this stage and see how it goes.
Hopefully the machining can be done this week and I can get it all back together next weekend.
Once again, brilliant write up. I wouldn't have attempted this without the info posted by everyone here.
I'll post more as I go along.