It appears yours is the 2nd gen t-maxx when they put the first 2.5 in it. It came after the first maxx that had the .15 pro engine on it. They made the arms a little wider, changed the towers, engine/pipe and throttle linkage. Probably other things, but that's what I remember. My first nitro MT was a t-maxx 2.5 from over 15 years ago.
I had that aluminum tank on mine and while it was durable, the cap would get galled from screwing it on/off over time and get hard to use, also as mentioned, you have no clue how much fuel you have. The stock tank was fine if you didn't run RPM shock towers that would flex easily and smash into the back of the tank. I ran an OFNA buggy tank on mine up where the throttle servo was and moved the throttle servo back by the engine.
The alloy trans case is nice, but not necessary if you stick with the stock engine. Not really necessary regardless what engine you run. I had one on my big block maxx, wasn't really any point to it other than it added rigidity to the chassis.
.21-.32 big blocks have the same footprint and all of them are too big for the maxx motor mount/space without a conversion kit. The engine is wider and longer by a decent amount.
An omega 21 (big block) vs an OS18CV-R (small block, same footprint as the 2.5/2.5R/3.3):
There were some engines made as "mid-block" sized that would fit in t-maxx/revo with the stock mount or they came with their own mount, but they were shorter so the engine didn't need more space front to rear. The OS21TM, OFNA Picco 21/26, RB TM323/TM523, Sirio made one too and there were a few others. Hard to find now though.
A 3.3 would probably be ok in a maxx in stock form, especially if you ditch reverse and get the forward only kit. Reverse was really weak in those things and to make room for the mechanics, the final output gear for forward was really narrow and had a tendency to strip.
I had a big block maxx many years ago with an XTM Big Block Conversion, which came with a chassis deck, chassis rails, cvd's engine mount, pipe and header. It was intended that you use the stock flywheel/bell on it, I can't recall if it came with a standard flywheel nut or not as traxxas's is different from everyone else on the 2.5/3.3.
XTM Conversion (I had this one):
Dynamite conversion (a buddy of mine had this):
Personally, I wouldn't put a 2.5/2.5R in anything but a smaller 1/10th scale car/truck like the rustler, stampede, jato or 4-tec. It really had no business being in a "monster truck" or the revo if you ever intended on driving it in grass. Unless you liked driving really slow. For me, even the 3.3 was a dud for running in grass in my 2.5 and 3.3 revo's.
With my first revo, I got an OS18TM which was a very punchy engine that would hold a tune all day long and was very easy to tune. It seemed very peppy in a short chassis revo, but pretty tame in a 3.3 chassis revo (longer chassis). I have an OS21TM in a 3.3 chassis revo now and it's not bad. I also have an LRP28S3 in a 3.3 chassis revo and it's pretty crazy.
That’s a good looking truck you got there. Congrats 
