i wanted to kno what brand i should get cause my car wont stop backfring and ive done the timing multiple times.

i found a timing chain with new timing gears for 75 bucks. its a billet brand are they good? i found it off ebay. it seyz its +8 to-8 on the crank adjustment and has 9 key locks on the small one and like 13 or so i cant remember on the big one with a dual chain thats pre stretched whats pre stretched? already stretched so it wont stretch any more?