RDIAS= registered drop in auto sear.
The block/pin is not just popped out. The way to remove is to mill the block out of the receiver. You vise the receiver up, then run an end mill close the pin side of inside of the receiver down the block. Once you have end milled past the retaining pin, then the block can be raised out, and what is left of the pin pressed into the cavity and remover.
The only way to replace the block is to buy a new block, then press a new pin into the receiver/block to hold it in place.
With the block removed, you are left with a hole on one side of the receiver. Most people will have the hole welded shut, then the receiver refinished.
Granted, some have just used a dremil cut off wheel to remove the block (cutting the block off close to the exposed pin) they are still left with dealing with the hole.
As for slowing down the 7.5" look into a 9mm SMG buffer if it's a car. You can slow the unlock down by using a pigtail to delay the bolt opening up.
As for your receiver, because you have SBR the receiver, pulling the block may be an option, but remember to look into having it re-weld/re-finished to lose the left over hole.
Bottom line is that a 9mm SMG buffer goes for around $40 and a pigtail gas tube a little more. The cost of removing the block, then having the receiver welded and refinished is going to run you in the hundreds.
Since the SBR is only a semi, modifying the receiver to use a M-16 bolt is wasted to gain the little of difference that the hooded carrier has to offer.
Note: since the buffer is a total waste on a semi rifle (bolt bounce has no effect on semi fire/bolt unlock), you could pull down a standard buffer and pack it with tungsten powder to weighted the buffer. By adding a softer buffer technology bumper end, the mass of the modified buffer will slow unlock of the bolt, and the softer bumper end will soften the blow of the buffer to the end of the receiver extension.