I wouldn't add arm days.  What you can do is add 3 sets of 10 of one bicep exercise and 3 sets of 10 of one tricep exercise at the end of bench and military press day.Also keep in mind the 5/3/1 schemes will likely seem easy to you because you come from a bodybuilding background and likely aren't very neurologically efficient.  However, stick with it and focus on being fast and explosive.  When you are doing squat, bench, deadlift, and military press, focus on bar speed, not muscle contractions.

I wouldn't add arm days.  What you can do is add 3 sets of 10 of one bicep exercise and 3 sets of 10 of one tricep exercise at the end of bench and military press day.Also keep in mind the 5/3/1 schemes will likely seem easy to you because you come from a bodybuilding background and likely aren't very neurologically efficient.  However, stick with it and focus on being fast and explosive.  When you are doing squat, bench, deadlift, and military press, focus on bar speed, not muscle contractions.

Haha! Read my mind on the seems easy at first glance, but I'll try to focus on the speed and intensity much more. I'm also assuming I shouldn't really worry about the Eccentric tempo like I do with BB (aside from not letting the bar kill me).

Sure you want to focus on eccentric tempo, you want to lower the bar as quickly as possible  😉Another note is notice I have another rounds of squats, bench presses, and military presses on their respective days.This does not need to be 5x5 every week.  Actually I recommend using a different set/rep protocol for each week.  Something like:4x86x44x68x3Another option is do paused reps for squats and bench presses or to use bands or chains.  For the good mornings though, I would stick to 5x5.

I have lower back issues as well and lowering the bar has helped me tremendously. It keeps me more upright with elbows pointed towards the floor, recruits more lower body muscle as well. Layne Norton mixes PL & BB routines and it odiously has helped him get crazy strong without the bulky powerlifter look. For the most part I'll follow MI40, time under tension, drop sets…etc. just to keep my joints from killing me but do throw in some heavy PL in when I'm feeling good.

I have lower back issues as well and lowering the bar has helped me tremendously. It keeps me more upright with elbows pointed towards the floor, recruits more lower body muscle as well. Layne Norton mixes PL & BB routines and it odiously has helped him get crazy strong without the bulky powerlifter look. For the most part I'll follow MI40, time under tension, drop sets...etc. just to keep my joints from killing me but do throw in some heavy PL in when I'm feeling good.

Yeah, Norton is definitely one of my idols... actually Physique Science Radio is what got me to go back to college for athletic performance this Spring. I'll dabble with the lower position just a bit since my quads will be firing enough to maintain their mass, which is really all I care about during this phase. Building over all strength is my concern. After 12 months of dedicated Aesthetic lifting, I felt ashamed of how weak I've become on flat bar lifts. My first deadlifts, bench press, and OHP in 8 months(coach switch) was from doing mi40x2 after my last show.

Make sure you base the percents off lifts you can do currently.

I'm aching for a fifth lift day. I feel stir-crazy only lifting heavy 4 times a week. Any room in 5/3/1 to add in a 5th day? If so, what outside of squat/bench/deads/OHP would be recommended to add? Maybe a second deadlift day? I like the idea of getting that back a little thicker. Maybe one day with Ham Accessories, one with back accessories? Thoughts from the experienced PL? Or should I just stick with 4 days a week.

If you do this, you will likely hurt your strength development. 

If you do this, you will likely hurt your strength development.

Unfortunately this is true.