That sounds pretty odd. The projection of the wheel studs are the same, front and rear, on the disc and drum setup. Unless the wheel has some wild offset - but I would think that would interfere more with the disc than the drum setup (overall diameter).
Did you get a staggered wheel setup? Different sizes from and rear - like the wheels they used on the MR2? That would possibly explain it.
As for stud length - you can punch those stud out and pull in longer ones. Not hard - you can rent the tool to push them out, then use a lugnut and stack of washers or section of steel pipe to pull the stud through.
If you are set on getting a rear disc conversion - couple of options.
- Get the rear discs off a JDM AE101/AE111 Corolla - those are a bolt on match to the USDM Corolla. These will be used parts, can be crazy expensive, given they have to be shipped overseas + typical JDM price gouging.
- Get the rear discs off a Celica, most are 5-lug, so you'll have to redrill the rotor, not rocket science, but definitely take it to a shop as it has to be dynamically balanced afterwards, or at least it should be.
- Willwood makes an aftermarket kit for the 7th and 8th gen Corolla. Think this was in the $500-$600 range, but they can be pretty hard to find stocked anywhere.