Yes, you will run into the same errors with the .tar.gz version.
There should be a kernel-source-x.x.xx-xx.x.x RPM somewhere on RedHat's FTP site. The x's should match what uname -r tells you.
Download that, and every time you update your kernel in the future (using up2date, I assume?), update the kernel-source with it, because you'll have to recompile your NVIDIA_kernel package at the same time.
Anyway, once you download that file, just rpm -ivh it. Then cd /usr/src/linux-<whatever version it is>/configs, and look around for a config file that matches your processor. Copy it to ../.config, cd .., and edit the Makefile. Change EXTRAVERSION=-xx.x.xcustom to EXTRAVERSION=-xx.x.x (i.e. get rid of custom). Then run make mrproper oldconfig dep. Then retry the rpmbuild --rebuild on the NVIDIA_kernel .srpm file.