Don't worry, because if anything fails, you can simply pull the plug from the new drive and your system will run as always.
Usually the motherboard has two IDE ports, primary and secondary. For performance reasons it is better to have the IDE hard disk on the primary channel and the CD-ROM drive on the secondary. In this configuration both drives have to have their jumpers set for: "master".
The biggest problem could be that your BIOS wants to boot from the IDE drive first. If your BIOS doesn't have a setting to allow you to boot from the SCSI drive first, then you can try two things:
1. Upgrade your BIOS to the latest version and hope that this has the desired setting.
2. Boot from the IDE drive. Install the NT boot sector on the new drive, using the emergency repair procedure and "repair" the boot sector. Copy the three to five required boot files from your SCSI disk to the IDE disk: NTDETECT.COM, NTLDR, BOOT.INI. You may have to change the drive figure in BOOT.INI. The main NT folder remains on the SCSI disk. If this works and NT boots, change the drive letters in NT such that your SCSI disk becomes C: again or whatever it was before the change, and give a different drive letter to your new IDE drive. Until you have finished that, don't care about the error messages while booting.
Make all changes such that you can still revert to the original system if all your attempts fail. As long as you don't make any changes to your SCSI disk, you can still always pull the plug on the new IDE disk and revert.
Please ask again if you need more details.