I think there's a bus from Porto to Salamanca on Sunday at 1000 (arrive at 1700)and from Salamanca to Valladolid at 1800 (arrive 1930).
see www.alsa.es (Oporto-Salamanca)
www.auto-res.net (Salamanca-Valladolid)
Spain is one hour ahead of Portugal, by the way.
I don't know how accurate it is, but you can check trains here:
[
reiseauskunft.bahn.de]
The options I found don't compare that favourably with the bus - the journey is either 11 hours with two changes or 7.25 hours with one change but leaving at 1715 and arriving at 0130 on Monday morning.
Kahoosier's Oporto-Lisbon flight appears to be full.
If anyone finds any better options, please let us know!