I happened to be surfing through and saw this - yes, my name is Paul, and I have an auto addiction.
Wilwood hydroboost? TYPO..!

Hydratech Braking Systems Model # 3023
http://www.hydratechbraking.com/GM_1stgenF_Body.html
The pics don't show it all too clearly, but I recognize that you have one of our latest systems using the improved 2011 Bosch production units! I'm curious to know what your feedback may be of the level of finish, quality, and overall preparation? You might not see it right off (especially in these pics), but there are over 6 hours of highly skilled meticulous show preparation that goes into each system during production. Something most people don't know is that I personally do all of the final assembly and preparation / detailing / order review, so your feedback falls right on the target.
Here are the answers to your specific questions:
1) The rubber boot is provided with the Wilwood MC in case you where planning on running this MC in a manual brake application in an underfloor mounting (where it might be exposed to road grime). It is not used in a power brake application (neither vacuum nor hydraulic assist).
2) There is no gasket, or need for such, between the master cylinder and the brake assist unit. The master cylinder is to bolt directly to the face of the brake unit - metal to metal.
3) This Wilwood MC design is dimensionally a '68-'76 C3 Corvette MC, running the early / deep GM MC pushrod spec. Wilwood provides that aluminum spacer bullet / slug with their MC in case you were possibly running a late / shallow GM pushrod spec. Our system # 3023 is set up for use with the early / deep GM MC pushrod interface, so the spacer is not needed in your installation. The above being stated, take the rubber boot and the spacer bullet / slug and toss it into your spare parts bin as extra unneeded parts with this combo. The Hydratech Model # 3023 is an out of the box bolt on "plug n play" for the Wilwood 260-8556 MC.
Here is some further related reading:
https://lateral-g.net/forums/showthread.php4?t=17060
You have one of our latest HIGHLY IMPROVED models, so let me know what you think.