It is very hard to secure a row of 5 seats to yourself--even if they do block it, that does not allow you to claim them as yours.

To answer the question---5 seats in a row is much better than a J seat. Place the extra blankets between the seats to smooth out the bumps and ask for a seat belt extender. The extender will allow you to create a very large seat belt that will not impede you in any way while lying down.