According to the AMA specs your 1968 350 should have five quarts plus what it takes to fill the filter. I wouldn't worry about being slightly over capacity, others might. Funny, the AMA specs for the 1969 350 is 5.5 quarts plus what it takes to fill the filter. I thought they were the same engine. Maybe the filters were larger in 68 than 69? I always put in a new filter and six litres, we can't buy quarts here anymore, but I think a litre is close to an American quart. You can find the AMA specs by going back to the home page then selecting the 67,68 or 69 page, then technical-AMA specs. All the stock specs for your engine are there.
The PF61 is not listed as a filter for the 68 Pontiac Firebird 350. It obviously fits but I notice it does not have a bypass relief valve nor anti drain back valve. The AC delco filter listed for your engine is the PF24 it also does not have a bypass valve but it does have the anti drain back valve. I use a NAPA gold, I believe they are made by WIX, and yes it has a bypass valve. Some do not like a bypass valve in their filters, some think dirty oil is better than no oil, but that's another can of worms discussion.
