There is plenty of good viewing in the GA sections in the circuit. If I were you I would have a wander round on Friday and Saturday and pick yourself a spot you like.
As far as getting to the track is concerned there is a very good bus service from Passeig de Sant Joan directly to the track. We used it last year and have booked tickets for this year. It’s a really good service and drops you right outside the circuit entrance. You can buy tickets on the day or book in advance to save time. The web site is http://www.sagales.com The cost to book online is €10 Euros return per day.
If its too far to walk to the Passeig de Sant Joan from where you’re staying there is a metro station Tetuan a couple of minutes walk away.
Hope this helps. If you need any further info just ask and I’ll help if possible.