Your 68 should have the same height engine bay as my 69. I had it 11 years, but sold it 3 months ago.
It had a stroker 351W with a Vic Jr intake. It fit under the stock hood. Barely. I used a 13" Ford Motorsport drop base breather (you can still get them for $35) and a 2.25" tall K&N element (part # 2865). I even removed the wingnut and used a bolt as the bolt head wasn't as tall as a nut/stud. I also made a torque strap so the engine wouldn't rock over and hit the hood when I nailed it.
I ran it without a spacer under the carb. No room. Later on I added a Boss 429 scoop and cut the hood under it. Added a spacer and a 4" tall K&N. Car ran better in the heat, but stayed the same in cool weather. Guess the only diff was the cooler air. Having the spacer made no difference.
A pic:
Later,
David Cole
<font size=-1>[ This Message was edited by: Dave C on 12/18/02 1:43am ]</font>