What about just the 866 cam? That's still for 8v engine, isn't it? And would a larger 38mm throttle body fit straight onto the spi unit?

yep you can whack an 866 cam in your head - is a great mod as it give significant increase in lift. You will need to check the valve clearances afterwards. EuroCarCare in Derby is a cheap place to get the shims.

38mm TB isn't direct bolt on no, needs some modification.. Best bet is to get a TB enlarged by sparky5 - i hear nothing but praise for the quality of his workmanship

I just found this back box on ebay, do you guys know if it will fit straight on?

http://i.ebayimg.com/00/s/MTIwMFgxNjAw/z/LL0AAOSwBahVT7E5/$_57.JPG

Mmmmm, looks like its kinda designed to go straight on but i'm not convinced it will terminate at the point one of the stock parts end - it looks too long ot just replace the back back section but not long enough to go all the way to the CAT section. Its hard to tell really just from that one pic but if you just search the fleabay for a seicento sporting exhaust you will see what i mean, doesn't have all the bends and looks shorter...

If you want something that will bolt straight on have a look at a cybox seicento exhaust - they sell them on ebay and i can't argue with the quality of the one i bought once upon a time. That or Supersprint rear box and centre section (they sell stainless and mild steel ones so be aware of what you are looking at).
Sportex is a bit cheaper than both but i don't think the quality is as good and they only sell coated mild steel and not stainless so it won't last like a cybox one which are all stainless..

edit: one of these http://www.ebay.co.uk/itm/FIAT-SEICENTO-STAINLESS-EXHAUST-SYSTEMS-/271847280999

I know what you mean, its missing the kink in the midsection, weren't most of the power flow systems custom made? Thanks though, I'll have a look!

all of them are custom made, powerflow is a custom exhaust franchise business - and thats why some are great and some are crap. I have bought a few cars with them on at time of purchase over the years and they have all been a bit crap really - all crush bends and not mandrel bends, one even had a creased bend where they had tried to bend it a bit further than the piece they used was bent. I affectionately call them powerslow myself - as do alot of people i know.