Feeney, who arrives from Tickford Racing, will be backed by Red Bull and Boost Mobile, while Mouzouris will run a pink and blue Pitbox.io scheme.
Triple Eight began running the #888 in the main game in 2004, with Craig Lowndes carrying it between 2005 and 2018.
Lowndes carried the #888 to six Bathurst wins, three of them alongside Jamie Whincup between 2006 and 2008.
The number dropped off the Supercars grid following Lowndes' full-time retirement, but it was brought back for the 2019 and 2020 enduros.
Notably, Whincup and Lowndes carried the number to victory in the 2019 Sandown 500.
It returns in 2021, albeit in the development series, with Feeney returning for a second season in the Super2 ranks.
Last year, the 18-year-old finished seventh in the standings with two top five finishes, and also made his main game debut at Bathurst alongside James Courtney.
"I think that the car looks incredible this year," Feeney said.
"To see 'Feeney' under the number 888 with Red Bull, Boost Mobile and Supercheap Auto as the sponsors is just surreal to me.
"I can't wait to see it hit the track and to finally share it with the public.
"Growing up as a kid in the sport, you dream of one day being associated with Red Bull, so to have them on my car, suit and helmet is a dream come true for me and an opportunity that I look to capitalise on."
Boost Mobile also returns with Feeney, having backed his Tickford Super2 entry in 2020.
"To continue my partnership with Boost Mobile Australia is unreal," he continued.
"I've been working with Boost for nearly two years now, starting from a sticker on my helmet, to driving in the Bathurst 1000 and now to supporting my 2021 Super2 program.
"I cannot thank Peter [Adderton] and Jason [Haynes] from Boost Mobile Australia enough for giving me the opportunity and to be able to work with such a successful brand.
"It has felt like forever after ending the season at Bathurst last year, but I have spent my time preparing the best way I can to return to Bathurst for the season opener.
"I have plenty of miles now at Bathurst and I can't wait to kick the season off. It has been four months waiting, but I'm ready to go racing now."