i only want to remove the plenum to get to my EGR valve, how do i do this? its 87 T/A with 5.7 tuned port 350

Cross your fingers
Usually the runner gaskets are bonded on tight to the plenum and the base with age
You can completely remove all the bolts and nothing will change because the gaskets are stuck tight
It requires you to at least loosen the runner to base bolts as well so you can tap the plenum with a soft hammer to crack it loose , then if the bottom of the runner moves,
it destroys the stuck on runner gasket which means the runner has to come off anyway to replace those

I have had cases were I had all the bolts out and hitting with a hammer didn't work so I needed 3 ft of water pipe into the front of the plenum in order to lever it free
Good luck

where are runner bolts? are they the torx ones?

yes. look at the top of the runners. they are right there

Pictured above is the typical set up for a 1986-89 MAF based system. Speed density is similar. (A) Intake manifold. (B) EGR valve (C) EGR temp switch (maf only) (D) PCV fitting, to the left is orifice for cold start injector (maf only) (E) Thermostat housing neck and bolts (F) Bolts needed for top plenum and runners- 13- M8x1.25x30mm, 2- M8x1.25x40mm, 4- M8x1.25x50mm, 1- M8x1.25x30mm throttle bracket stud, 20 total, these use a T40 Torx head bit, NOTE these are metric. (G) Fuel rail typical for a "F" body, maf system, fuel lines for Corvette is on opposite side. Look at picture, as this is a complete system (H) cold start injector and fuel tube (maf only) (J) Fuel injectors-8 total (K) Intake runners, speed density will not have the cold start opening (L). (M) Intake air sensor (P) Intake plenum, with throttle body, be sure to get complete throttle body with no missing upper and lower cover plates, bottom plate has provisions for IAC and coolant tubes. (Q) Throttle bracket and bolts (R) Vacuum fittings for fuel regulator, heater control and brake booster, speed density will have a fitting for MAP sensor and mounting boss.

Can anyone tell me the size of the three tiny screws needed for the plastic decorative piece on top of the plenum? Desperately looking for these 3 little screws!

I think they are M4. Couldn't find them in Torx head however, had to settle for Allen head.