Valve size ???

[A Snake]

A little insight here and I'm not trying to rain on your parade, just give you something to think about. There are two reasons why 427 sized valves won't work on a 390. First, the medium riser heads have the valves spaced farther apart in addition to the larger diameter. the spacing is BECAUSE of the larger diameter. Second, in spite of what you may hear, you WILL hit water if you try to notch the 390 cylinder bore enough to clear the valves....especially with a high lift cam.

Now, unless you wanted to try for 9,000 rpm, the bigger valves won't do anything but steal your low end torque.

As previously posted, the 428 CJ valves help quite a bit....especially if the heads are ported by a pro OR you by the aluminum Canadian CJ heads which already have the port work done.

Can you make 450 rear wheel hp with a 390? Yes, but you won't want to drive it on the street. That takes at least 11:1 compression or more and a really stout camshaft along with a large (850cfm) carb. Definitely not a daily driver type engine. Racing fuel only and changing spark plugs like you change socks and shorts....because the rich idle needed for the lumpy cam along with low rpm driving fouls the heck out of them. Don't forget, with that wild cam, you'll need some stump pulling gears like at least 4.10:1 or lower.

Just some thoughts. Frankly, if you have a 390 block, go find a 410/428 crank, put the CJ heads on it with a decent manifold with a good half inch lift cam and prepped carb. You'll probably make 400 hp and still be drivable with a nice rumpty rump idle. Frankly you'll love it and a Viper still won't be able to catch you!

Al