Wrong info there TRAVAGLIANTE
The only seats that will fit with no modifications are determined on the year of the car. Meaning:
If you have an '88/89 Civic HB, you can only put 88-89 HB seats in w/o modifications. CRX SEATS ARE NOT INTERCHANGABLE W/ THE HB. Modifications will need to be made and vice verse with the CRX.
If your car is a 90/91 Civic HB, you can only put 90/91 HB seats into your car w/o modifications. Again, CRX SEATS ARE NOT INTERCHANGABLE W/ THE HB.
I myself did put some '89 SI HB seats into my '91 std. HB. I had to do some modifications to make it work though. Search for my thread if you want to know more.