Whacha talking about Aspen? I've run several 2.5, '99+ trannies on my 1.8 and I mated my 1.8 FWD tranny to one of my 2.5 engines (the phase-II DOHC one). Everything fits just fine. You can drop a 2.5 into a 1.8 FWD car without changing much - aside from the ECU, which requires wiring harness which is a pain in the butt. Nonetheless, mechanically, all interchangeable.
In fact, when I could not find a clutch alignment tool once, I took apart my 1.8 tranny and used the input shaft as an alignment tool.
HEaviest alignment tool you've ever seen!
I think the 1.8 tranny will handle a stock 2.5 no problem. I think once you start getting more power out of the 2.5, you'll start having problems... But even basic boltons would be okay... The 1.8 is very very similar to the 2.5 tranny. And the trannies themselves are identical in design with the difference being the terminator on the FWD models instead of the center diff (and thus a smaller rear tranny cover).
As for the weight - I don't think any impreza is under 2400 stock. The '93 4-dr is the lightest, followed by a tie, I think, in the '94 4-dr and the '95 2-dr. THe '95 4-dr is like 5-10 pounds heavier yet. ANd after that they start porking up pretty quick (I mean look at the RS - 290 pounds heavier than an AWD converted '95 2-dr.
Joel, who thinks that the hollow gear drive shaft design of the scooby tranny is really cool for AWD but is just a total waste in a FWD car and would only serve to contribute to more driveline loss than a civic has.