Tasco is an OK cheaper scope. Did I read that right- 10-40X? Lord, that is gonna be pretty dim on high mag, and hope you are on sandbags (or don't get coffee jitters) Most .308s are midrange recoil (but I did just get a 6 lb carbine in .308 that will put tears in your eyes!) What are you planning to use scope FOR? IMHO, high mag does not work well for hunting except very small varmints that stand very still. Paper punching, different matter. Will the 50 mm bell clear the barrel OK?
What we have here is... failure- to communicate.