Would it be possible to get the 14" rotor and caliper relocation bracket for a GMT900 and install it on a GMT800 using the GMT900 front caliper and GMT800 kit for the back?
No, the hat size for the two rotors is about 2 inches difference.
FRONTS:
The GMT800 front brake setup that you want to replace is your 12.8" rotor with a single piston caliper. The upgrade from the GMT900 is a 13" rotor with a thicker (better vented) rotor, but it has a dual piston caliper with a bigger pad.
GMT800
EARLY GMT900
LATE GMT900
The 14" rotor is from the SSBC kit, or perhaps you could search and I would guess someone has already tried, but the K2XX (2014+) model Silverado has a 14" front rotor, NO CLUE if it fits, but if anything OEM does, that's going to be it.
I had this saved on a notepad. if this helps to expand your search. The larger 13" rotors came as early as 2005 on Silverado and sierra depending on trim. You just need to get the pads that match which ever set of calipers you get.
2008-2013 SILVERADO 2008-2014/2016 ESCALADE/XTS AVALANCHE/SUBURBAN/TAHOE/YUKON/EXPRESS 1500/SAVANA 1500/
2005-2006 SILVERADO/SIERRA 1500 EXCLUDING: Hybrid, SS, HD, & rear disc; Had the larger 13" rotors.
2007 SILVERADO/SIERRA 1500 Had the same 13" rotors but had the same earlier pad # as 05-06 (not sure if thats classic only).
- YEARS ---- PAD # ------ MODEL ---------------------------- YEARS ----- PAD # --
2005-2007 - D1092 - SILVERADO/SIERRA 1500 ---------- 2008-2014+ - D1363
------ 2007 - D1092 - AVALANCHE 1500 ------------------- 2008-2013 -- D1363
------ 2007 - D1092 - ESCALADE (EXCL EXT & ESV) ------ 2008-2015+ - D1363
------ 2007 - D1092 - ESCALADE (ONLY EXT & ESV) ------ 2008-2014 -- D1363
------ 2007 - D1092 - SUBURBAN 1500 -------------------- 2008-2015+ - D1363
------ 2007 - D1092 - YUKON 1500 (INCL XL & HYBRID)-- 2008-2015+ - D1363
------ 2007 - D1092 - TAHOE ------------------------------- 2008-2015+ - D1363
------------------------ EXPRESS VAN 1500 -------------------2009+ ------ D1363
------------------------ SAVANA G1500 ------------------------2010+ ------ D1363
REARS:
The GMT900 has a larger rotor (13.6") than the gmt800 (13"), but for some reason its a single piston caliper. That's the reason to stay with the GMT800. Just not worth it, until someone comes up with a better caliper to use on them.