To be successful with wieght lifting and boxing you have to know the balance the balance of when to
1) Go heavy or

2) Go explosive

For boxing you wanna work alot of Fast twitch muscle fibers like a sprinter has as a opposed to a Power lifter who has alot of slow twitch muscle fibers

Fast twitch muscle fibers are episotic muscle fibers that are able to generate an explosive amount of power very quickly, fast reaction and generate speed.

Slow twitch muscle fibers are also explosive but not as fast reacting but are more for endurance and and generating power in a gradual rather than explosive way

To Get fast twitch muscle fibers you need to lift high reps explosively not slow in motion which generally makes your muscle grow.
So if you wieght lift slow and controlled it works best for size
If you lift faster and more explosive it works for strength more effectively

for boxing you should do alot of explosive workout training like Squats, dead lifts, Power cleans and snatches, Lunges etc

I would recommend shrugs and upright rows for your Traps (HEAVY) , Shoulder presses(heavy), Lateral raises, front delt raises, rear delt raises ( all LIGHT wieght)
ALot of training for Back so Lat Pulls( heavy) Bent over Rows (Heavy), Tricep training because they extend you arms so are very important in punch So, Dips, Tricep Extension etc, Biceps are used for pulling so there not so important in Boxing more beach muscles

If you wanna keep your speed while lifting make sure you stretch alot and do alot of SHADOW BOXING Do ALOT of rope training IF you follow this I bet youll Be STRONGER and FASTER than EVER.