Keith,
You’ve received a lot of good responses here. I’ll add mine to the pot as well. Like you, I have a 351C with 2” primaries. And this is a big problem. I am preparing to build a new set of headers (I dropped the engine and moved it back, so an off the shelf set from my manufacturer won’t fit) that are 1-3/4” to improve port velocity. 1-3/4” diameter primaries should be fine for 450 hp, which is my goal in the end. I realize this is not a cheap or easy solution, though it’s probably one reason you’re motor seems sluggish down low.
As far as the 4V heads—especially the open chamber ones—you don’t really have a big enough cam, displacement or compression to use them. The ports and valves, as others have said, are simply too big. They work well with strokers, 10+:1 compression and high rpms. On a relatively mild engine like yours, the large ports give low vacuum signals at low rpms, so it’s difficult to get low end driveability. Jeff gets away with it because he’s running an awesome indepented runner induction system, and Tim was able to improve his low speed tractability by having his carb tuned to so it’s booster venturis are more sensitive. This is a good place to start, as it sounds like Tim has had good results with a similar setup
The port plates will help, but they aren’t the answer, but rather (in my opinion) a band-aid. If it were me, and I always try to do things on a budget, as I’m sure you are trying to as well, I would suggest the following. Swap or sell your 4V heads for a nice pair of 2V heads. Do the same with your intake. Since someone buying the 4V parts would be “upgrading”, you will probably come out slightly ahead money wise, so it’s not an expensive proposition. If you want to spice things up a hair, get a pair ot 2V heads that have a little cleanup on the intake ports, but leave the exhausts alone. Or better yet, buy a pair of 2V Aussie heads which have ports slightly larger than the US 2V heads, but have a quench combustion chamber that raises the compression 1 point without any problems with detonation. Edelbrock, Weiand and Holley all make nice 2V dual plane intakes that will work well with these heads and you can pick them up for between $100 and $150 used.
Here's an interesting discussion of the port plate you might find insightful:
http://www.network54.com/Hide/Forum/...eid=1014127496
Finally, I would go with more cam, something similar to what Tim is running or maybe something along the lines of the Crane H-290 (formerly Fireball II 290) cam:
http://www.cranecams.com/master/apps/ford37.htm
I think there is still a lot of tuning left in your current combo, and that’s where I would start. I just stated the above as food for thought. If you want an engine with low end torque and moderate HP at the top end, 4V heads probably aren’t the best choice. If you are willing to sacrifice a little torque down low in exchange for some more HP up top, you can do it with the 4V heads, but you really need more compression and cam to make best use of them. But as others have shown, you can more than likely reach your target of 350 with a cam swap and some tuning. I think you’ll be giving up a noticeable amount of torque with the huge headers though. So my opinion would be to try installing the exhaust port plates, verify your cam timing with a timing wheel to make sure the cam was installed properly (or swap in a bigger cam if you wish), borrow a 600 or 650 cfm vacuum secondary carb and play with your timing. You need a way to quantify the results though, so a G-tech or other type of performance measuring accelerometer might be a prudent investment for the $140 they run. Please keep us posted and let’s see what we can do!