Towing to Portmerion (Porthmadog) - best route?

Traveller65

I'm working in Portmerion next week and will be driving from Monmouth with my caravan. 



I've looked at the map and there seem to be a few arrowed roads on the way there. Has anyone done this and can they offer me any advice? 



I'm wondering about going up the M5 to Stoke on Trent then vis Oswestry, or is it ok to take the A470 all the way?