The 33% number is just a guesstimate that someone made years ago and put into a formula. It isn't a hard and fast rule. Some people think it's gospel and your smoker will run like s*** if you don't follow that - nothing could be further from the truth!
I built a 60-gallon tank out with a 30 gallon firebox - people said, "Oh that firebox is WAY too big for that smoker, it's going to be way oversized and isn't going to run well." BS. It ran awesome. Great little backyard/patio smoker!
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Conversely, I've run my friend's big pit, which is a 230-gallon, with a 32-gallon firebox, which should be drastically undersized according to the 'conventional wisdom'. Now, it took me a little while to figure out how to run his pit best, a couple of cooks, no more. It's dead simple now.
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Bottom line is, you work with what you've got. You can make anything work, the key is good airflow and minimizing obstructions, nice big throat opening and decent sized, plenty long exhaust stack to help generate draw.
However, all that being said - I'm with
seenred
, I'd suggest keeping your tank whole and waiting, looking and being patient to find a piece of pipe to use as a firebox. It'll most likely be thicker walled AND you won't be cutting down your chamber and having to cap 2 ends and you will never regret having more space. What you're planning now, I think you may wish later you had a more than 20" deep firebox (for managing splits, etc) AND you may someday wish you had a little more space in the cook chamber. You will never regret keeping it full size, I don't think. You MAY regret cutting it down later. I know 120-gallon and 150-gallon tanks around here are rare as hen's teeth.
Anyways, that's my thoughts.
<edit> The caveat to that is a reverse flow smoker - they seem to be much more 'iffy' on firebox size. I have not got experience with those personally, so if that is your plan, I'd talk to someone who has built a bunch of those.
