I thought someone with a little more expertise might have been able to help you. You need to do better research than a post on a forum. Misinformation off a forum is not going to keep you from committing a felony. The "Billy told me it was okay" defense will never be a viable defense if you fail to fulfill the requirements of any NFA item. As they say ignorance of the law is no excuse.
My understanding is any barrel length under 16" -OR- a rifle overall length of 26" requires a NFA stamp.
A 9½" barrel with an 8½" (removable) suppressor is still an 9½" barrel. Permanently weld the suppressor on then it becomes an 18" barrel. If then the overall length is less than 26" it requires a stamp. If it's over 26" then you're good to go. It can be adjustable but the shortest possible length has to make the overall length over 26".
As far as I know the form 1 is what you need and $200 in your pocket and oh yea jumping through the hoops then at the end the $200 won't be in your pocket anymore but you will be the legal owner or an SBR months later.
DISCLAIMER: I have never attended any law school and have no legal training whatsoever and have never been licensed to practice law anywhere except on an internet forum.