How hard is it to remove the drum brakes and replace them with disk brakes? And should I do both the front and rear?

You will need completely different parts to do a change over. Its not so simple. You may also have to replace the differential/rear alxe if there are no kits available for conversion.

Yes, there are kits available. Check online and go look at some of the differences.
Like: http://www.google.com/#hl=en&source=hp&q=67+chevelle+disc+brakes&rlz=1R2GPEA_en&aq=f&aqi=&aql=&oq=&gs_rfai=CwYF4c_Y5TJnzEpG2zQS0k6T_CwAAAKoEBU_QuzaW&fp=4b53d993194b88d
For the front, you will need to replace the spindles and in the rear, special brackets (if you don't opt for the whole axle conversion).
You will get special hoses and tubing so be ready to fully bleed the system to eliminate trapped air.
This is a vey big job. Are you tackling this yourself or do you have some skilled help?