quad cam ej25 will fit. I have done one myself to a previous car, started life as the stock ea82, then went for ej22 and followed it with an ej25. You'll have about 20mm clearance between the chassis rails and rocker covers, but thats enough.
I used an adaptor plate to the L series dual range box and it worked alright. however i destroyed 2nd on my first outing (box had a good thrashing prior to that) put another box in and it went good for ages.
things to consider for conversion;
engine get a med KM engine for about 1500 i reckon
wiring - ensure you get the wiring loom and ecu
best to get someone who knows what they're doing to strip the loom to the bare minimum to run the engine/ splice the necessary bits into the L series loom
Custom radiator - the stock L series radiator wont cope, i spent about $500 getting mine modded to a nice dual core, 2 decent thermos and you should be right
L series crossmember mods, some just elongate the engine mount holes and bash the back of it with a sledgie, I made new plates for the engine mounts, changed the angle to suit, raised it about 15mm.
Exhaust, will need doing to match up to ej25 headers, altough i ditched the ej25 set up and used my previous ej22 headers... much easier
Fuel supply, will need to install an external walbro pump, might as well fit a surge tank in the process.
all up as an estimation, to do the majority of it yourself, i'd set aside about $3000.
It is a worthy conversion if your attached to an L series, but in my opinion, your better off getting an early model outback, same engine, better car feels much tighter (offroad) and eveything is designed for it.
here's a pic of the ej25 in the l series... you can see my surge tank and fuel pump set up there too, can be done under car as well