The ledger board that I installed for my cathedral ceiling (flat part) I nailed. The Inspector required it to be lag bolted
I would install a board going across as a "ledger", lag bolt it in & nail it - if that make you feel better
Then land the 2x8's on top of this board for support
2x8's should work for that span, depending upon the wood
I woud not use a 2x ledger board
Span calc:
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp
If you're existing garage wall are open line each joist up with a stud
Nail each joist to the side of the stud & toenail to the ledger board
I would install a board going across as a "ledger", lag bolt it in & nail it - if that make you feel better
Then land the 2x8's on top of this board for support
2x8's should work for that span, depending upon the wood
I woud not use a 2x ledger board
Span calc:
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp
If you're existing garage wall are open line each joist up with a stud
Nail each joist to the side of the stud & toenail to the ledger board