No need for you to rent gear, especially as the job is unpaid, a favour etc.
A few things you can do..
Keep the 50mm at 1.4 or f2.0.
Shoot ISO 800 but use RAW and use DPP or ACR carefully (chroma noise NR as far as you can push it,followed by slight sharpening where required) This should be no problem for the XSi at 800, if you do some work in RAW.
Shoot manually. When you are using the 17-40 your exposures are going to need to be 2-3x longer than with the 50mm. So I would recommend a monopod, or one of those beanbag pods you can rest on the back of a chair.
Use the exposure scale as a quide, but remember, some of the shots are just going to be dark, and should be dark, so make them dark.
If in doubt, bracket. Shoot lots.
Use live view, use MF with live view zoom.
Use WB presets. Tungsten will be best before the lights dim, the projector may be daylight balanced, may be tungsten balanced, a couple of shots will confirm which. And even if you get it wrong in cam, it's easy to fix with RAW.
Your XSi is up to the job. Yes a 5D3 would be better and a 1DX better again... but they are unncessary.
RAW can save your bacon here. Digic 3 at ISO 800 will be absolutely fine.
I get annoyed at these folks prompting you hire the most expensive and greatest gear.. you are looking at at least two or three hires to get used to them... they are very different to operate from an XSi. Also why would somebody tell a novice to hire two unfamiliar bodies? Defies understanding.
A lesson I learned ages ago was plan your shoot and carry the minimum required. The more time you spend fiddling and swapping,the less time you spend shooting.
Go in with a plan, all the kit you absolutely need and no more. You'll get better results with the XSi you know how to work than with a 1Dx that will flummox you if you've never used that style of body before.