7th floor would have been a good  place  to assemble the rifle but Oswald would have to bring the package from the 1st floor annex part of loading dock where he hid it that morning after entering about  30 secs ahead of  BW Frazier.

How do you know where Oswald hid the rifle that morning? Sounds like you are just guessing.

So Frazier saw Oswald  carrying a package go thru the 1st door that’s part of the annexed bldg that’s attached to the TSBD and then Oswald had 30 seconds to place the package somewhere (an open
Storage barrel perhaps ) unseen by Frazier doing so.  Then when Oswald  opened the back door of TSBD, Dougherty saw nothing in Oswalds hands.

Pure assumption. Just because Dougherty didn't remember seeing Oswald with a package doesn't prove Oswald didn't have a package. People tend not to remember mundane details like that because they don't notice things like that. I would have to guess where Oswald hid the rifle that morning too, but i bet I could make a better guess than that.

So anytime from 8:30 to 11:30, Oswald could possibly have gone into the annexed garage bldg probably using the rolled up door beside the 1st floor staircase , retrieved the package and he either took it up to 7 th floor using the stairs or by using one of the freight elevators.

The possibilities for where Oswald hid the rifle and when he assembled it are almost limitless and anybody who thinks they know the answers is just guessing. If I were in his shoes, I would find an out-of-the-way hiding place and wait for everybody to go to lunch before assembling the rifle, then find a secluded spot with a good line of fire down Elm St., but Oswald wasn't obligated to do what I would have done. He got to make his own choices. I'd say him made some pretty good choices, even if I don't know what they were.

Unfortunately I dont think There were any witnesses that have WC testimony that support this scenario and given that Dorothy Garner on the 4th floor probably did NOT wait 60 secs post shots to follow behind Adams and Stiles and given BW  Fraziers 24 in estimate of the length of the bag, this hypothetical scenario is basically irrelevant.

Almost all fill-in-the-blanks guesses are irrelevant because there is no evidence to support any of them. Only Oswald knew when and where he assembled the rifle and he's not talking.

I just threw it out there as an LN option since most LNs are convinced the bag probably contained the disassembled MC rifle.

If Oswald didn't smuggle the rifle into the TSBD in the 40 inch bag that had his palm print on it and fibers matching his rifle blanked, it just means he brought the rifle into the TSBD at some unknown time. I find it far more probable that Frazier simply misjudged the length of the bag than that Oswald brought his rifle into work at some other time.

would be beyond out of proportion and far beyond innaccurate.

Norman’s total time for the 3 shots is about 5 secs ,  but he spaces them more equally apart over that 5 second duration.

The Z 313 shot has possibly 4 different sounds that occur so close as only 0.5 sec or less:
A. The “boom” from the muzzle of the rifle
B. A “Thud” sound from bullet hitting the head
C. The metallic clang sound a fragment hitting the chrome frame of the wind shield or rearview mirror
D. The sound of the windshield being cracked by a fragment.

Clint Hill, who heard the third shot as he reached the rear of the limo, described the shot as having a metallic sound. That might have been due to the fragment hitting the windshield frame. Because the bullet was traveling faster than the speed of sound, he would have heard that before he heard the muzzle blast.

We all know about JFK reloaded but most documentaries use private proprietary made 3d Maps of Dealey Plaza, not ideal.
SO...if we were go near this, how would we achieve a computer simulation?

Were the windows open in that little building of his? Makes me wonder how clearly he heard the shots.