On V6, or V8, engines, which sit crosswise in the engine bay, it is often difficult to access the spark plugs next to the firewall (bulkhead).
Some repair manuals instruct to remove the intake plenum, etc. Removing the plenum has the potential, upon re-installation, of creating a vacuum leak from poor sealing, trapped wires, etc.
I have found it best to go in from the end of the engine (the side of the engine bay). Often, the alternator and it's bracket, or a power steering pump (it could be a another component) have to be removed to reach between the engine and the firewall to the spark plugs. I found this method to be quicker, and easier.
On some vehicles (Chevy Venture, for one) the engine has to be pulled forward, In addition to removing parts, for adequate clearance to the spark plugs. The engine is supported with a jack (or block), the front motor mount is removed, and a ratcheting tie-down strap (available from Walmart) is used to pull the engine forward a few inches.