You ceiling mount the projectors and can use first surface mirrors to shorten the throw distance. There are several simpit tricks that have been developed over time.
As far as curved screen, yes you can use a curved screen but it's a bit tricky to get the focus points on the projectors just right. Also a big issue with multi-projector is getting the edges lined up perfectly or using software to blend the edges between the projections. I won't have to be quite as picky about lining up the the projections though with the SS1 build I'm doing, because it has blind spots where I can hide the edges between the windows field of view.
There is a new app out in alpha testing for simpit building that can pre-warp directx output to the shape of your screen. It's called Sol7. They have a hobby simpit version of thier software that is targeted at people who use a TH2G or DH2G surround projection setup. It's pretty pricey at $430, but nothing compared to the commercial version that is $4000+. Right now I just do it the hard way and manually line up the projection edges.
My projectors I got on the cheap. I can't really drop $430 for Sol7 atm. I got my first projector as a factory refurb and the other 2 as "as-is" broken projectors on eBay. Being naturally curious, I was able to diagnose what was wrong with the 2 "broken" projectors from my original working one. One was merely a burnt-out bulb and the other had something jammed in a fan forcing an overheat error. Anymore you can get cheap 1000+ lumen projectors on eBay.
Also, don't worry too much about the "expensive" bulbs because there are cheap DIY replacements for those too. I know of a source that caters to AV forums where you can buy the projector bulb itself for $65. Just remove the old bulb and mount a new one. It's a small pain to tare down a projector bulb refactor cage and dig out the plaster holding the old bulb and putting in a new one, but $65 sure beats $350 every 2000 hours IMHO.
Frankly, a single projector would be well suited for 99% of the people wanting an immersive simpit build. I just went a bit further in my setup because I like to experiment with things.