Monkey Boy is right.
Don't try porting the plenum, just get an aftermarket one.
The cam/valve springs make a huge difference too. As Monkey boy says, if you do some thing you definitely want to change the cams too.
Only cams, or only heads won't help a lot.
If you want to go thru all the hassle of porting heads, you want to do the following things too:
- K&N FIPK
- Larger throttle body
- Aftermarket plennum
- Port the heads (or go with ported PI heads / intake)
- Get good cams/springs
- Get long tube headers
- get a high flow exhaust.
- get a programmer and maximize output at higher RPM (advance spark)
(all those mods will make your motor become a high screaming RPM hugry beast)
Talking about the head porting:
I know that in the past some people got into a huge argument over air flow and heads (people got banned).
Since I've never ported 4.6 2V heads myself, I can't realy say a lot.
But as always, the biggest improovement in air flow is not being done by polishing the intake runners, they are already a lot larger (cross section) than the valve pocket.
Cleaning up the valve pockets is always a good idea; especially around the valve guide. The transition from the runners into the valve pocket into the valve seat is a key element too.
Looking at the combustion chamber, you definitely want to smoothen out that turbulence vane and the valve seat.
I don't know how much you like to do things like that - I love it.
If I would be you I'd just get me a set of used heads and do exactly what the guy did in the article below. +20% on the intake and +15 % on the exhaust sounds bretty darned good to me. http://mustang50magazine.com/techarticles/23018/
Have fun porting!