I'm eyeing off some 17 X 8" second-hand rims to use for the track (I have been using 16X7" rims, want more width). I know 8" wide rims fit with no problem when they are +35 offset, but what about +40? Will they foul on the inside with 235/45/17 tyres?

I'm almost certain they will fit with no drama, but I thought I'd double check. So is anyone running 8" wide rims with +40 offset?

my 20s had an offset of 42, and were 8inch wide. So the rim wont have a problem touching at all.

Hopefully someone else can vouch for the 235s

I think you will be fine but use this site to compare the pair:
http://www.wheelsmaster.com/rt_specs.jsp

iv got 18x8s offset is +38, my car isnt lowered yet however it does look like my front wheels do stick out a little bit from the guards.