if this is the stafford b-24 the 9/5 will be fine you will find for scale flight you will be at 1/2 to 3/4 throttle.i have the old os 25's pre schnurle on 9/5 props at 13.5 pounds flies fine off of grass after 20 years.my tanks are all mounted right behind the firewall,4 oz per the plans with no problems using fai fuel,no nitro.sound to me with the 3b prop you were getting a vibration problem,specially if you could see the prop hub move,either due to the large hole in the prop,my guess ,or out of balanceprops.prop should fit on the shat without any side play at all.if there is vibration will occur as you cannot tighten the prop centered.