340 Cam, rocker, valve spring questions

Where to start and in what order.

First you need to calculate your actual static compression ratio. Assuming 10:1 is correct, you are going to want a cam around 221 duration at .050. Advertised duration will not really tell you how the camshaft will act. You need to concentrate on duration at .050 on the intake side. If the head hasn’t been altered around .500 lift is where you will be safe at with a true 1.5:1 rocker ratio. Also, J heads, unless they are heavily ported, will flatten and lose flow after about .500. Lift. 550-.570 Lift if they are heavily ported, bowl worked, unshrouded, larger valves etc. Yata, yata.

The radical sound of the cam I believe you are wanting comes mostly from valve overlap/ valve separation. A tighter lobe separation will give it that drag car cammed sound, but at the loss of power curve. Like 107 for example, This is good for a drag car because properly set up you would be working within that narrow power curve, a 112 on the other hand would sound more mellow but give you a much more broad curve, better suited for street.

So I’m thinking you want a cam around 221 at .050 intake, and just short of 500 lift. Probably around 108-110 separation. Going over 500 lift won’t really help you with J heads. That is “IF” you verify that you are for sure 10:1 static compression. Truly is best to call a custom cam grinder, it won’t cost much more and will work better for what you want.

Once the engine is complete, the correct way to figure out stall speed is to break in and dyno the engine before install, find out where your torque curve comes in at, then select a converter that flashes about 200 rpm below that point. Again this would be for a strip oriented setup. For street I personally wouldn’t go over 2500-3500 stall for drivability reasons, not saying you couldn’t go higher, just depends on you and what your willing to put up with in stop and go traffic.

Gear size comes into play once all that is finished. If you want a max effort 1/8, or 1/4 car, you will need to select a gear that will have you at max horse power rpm when your going through the traps, that goes back to the combo and dyno results. Just a quick figure in my head, 10.1 static, with 221 duration at .050 and just shy of .500 lift at a true 1.5 should have you somewhere in the 5800-6000 rpm trap speed range with the correct gear. Probably 4.10 and up, depending on tire size and assuming 1.00 final drive ratio. Again strip setup. Street you would want the gear that will give you the best of both worlds, but emphasis on highway speeds if you intend to travel on the interstate. Just speculating that would probably fall around 3.55 to 3.73 for your combo, depending on you.

So to directly answer your question, the cam choices you are looking at will all work, but none will work as well as having the right cam for your combo custom ground. Factory Mopar rockers are around 1.45:1, I don’t remember if those 273 rockers are true 1.5 or not, someone else will chime in on that. A 4500/4900 stall converter for a street car is not something your going to like in stop and go traffic. But remember there are loose converters and tight ones. Some will flash to stall before they even move, some will gradually come in. But that’s another conversation. Long winded post but wanted you to have as much info as I could at this stage of your build so you could get the most out of it. A 750 double pump race carb would be a good choice for a hot street setup. Might also consider a street avenger 780 vac. Or if you can score a original 340 intake there is a 1000cfm thermoquad competition carb on eBay right now for less than 400 with the choke horn removed. Good luck buddy...