alfa speedline alloy wheels

I have just purchased some 16" speedline (5 hole) alloys for my 156
(from ebay) and would appreciate a bit of advice. Basically the wheels
didn't come with bolts and my existing bolts (my car currently has some
fairly nasty 15" non-alfa alloys) are slightly longer (by about 5mm)
than the ones the previous owner was using. Can anyone tell me what
length/specification bolts I should use and if my slightly longer bolts
would be a problem to use.

As an aside, does the extra size (my previous wheels are 15") affect
the speedo readings significantly?

thanks!
Tim