You need torque.
go with a 351 stroker and BUILD your own headers. Not as hard as you might think. Its however Very time consuming. I originally bought vintage NOS Mustang II headers for my V8 conversion. Was told that they fit fine. Ya well if i wanted the distributor poking thought the hood. So i went about putting the engine where i wanted it and then modified the headers to fit better.
I started by getting some mild steel 1.5 mandrel bends from Jc Whitney about $70 worth total. then i started to cut and paste where i thought i needed.
Yes i made a few mistakes along the way and it took way longer than i thought but the results were incredible ( to ME.)
I'm not a fabricator by trade (I'm a machinist) but if your just wanting to get the exhaust out with some efficiency and not looking to be able to spend 1500+ for custom headers this can be a way to go. Keep in mind however that they wont have the HP potential that a Pro would get but your talking about needing torque anyways.
Also theres the "I did it" factor. Being one of the few that tackle such a project can give you some great satisfaction when your done.
While the cleanest way is probably to start with brand new components , I found it way easier to have something to start with. Your existing headers come to mind. They are already paid for and have the same flange to weld to. Trouble is that they are probably 1 5/8 and you'll probably want 1 3/4 with a 408 and up. But i bet you could get by with them and still build 500 hp and 550lbs TQ with the "small tubes". ( Feel Free BIG JOE to chime in!)
Having said that you still have to basically cut apart the entire set of headers even if you'll only replace 1 tubes because you cant just weld around 90% of the tubes that run side by side as they feed to under the cab. You
can weld around and near the head area that has the clearance. I took mine 1 tube at a time.
Of course you have to be realistic on your place to work, supplies that you already own, cost of things needed that you don't have and you might not use again.
My parts cost:
Mandrel bends, 180s, 90s, straights, Collectors, flanges.
Given what i spent on my parts id say was around $150 for header parts, $250 for the headers, $150 buddies "old" Lincoln Welder, $10 for shielded wire, $100 for the grinder and assorted wheels, $30 for Sawsall blades
And figure another $100 for the stuff that you just always forget and nickel and dime you to financial death.
Supplies you might need.
1. 4 1/2 in angle grinder with plenty of cut off wheels,assorted flap face grinding disk wheels and a wire
2. hooker headers tube weld sleeves.(makes the job much easier, wish i would have known about them when i did mine. Also will prevent most of the slag from building up on the inside of the tubing)
3. Wire feed welder, able to weld 16 gage or thinner. TIG is best ($$$$) but if all you have is a mig then that will work, Just be prepared to be smoothing the welds alot.
I used my Mig with shielded wire. No gas. I just couldn't afford it. I ground alot of splatter off.
4. Mapp gas torch. So you can finish form the collector around the 4 tubes. Also helps to make slight adjustments to the finished setup if say the collector is pointing 3/8" at the wrong angle when your done welding the assembly together.
5. Engine hoist with a load leveler. So you can leave the engine and trans together. Building the headers off of the engine and then test fitting the assembly about 39 times during the course of the building project.
6. Electronic welding helmet. INDISPENSABLE for "backyard" welders like me. I couldn't weld myself out of a tin can without mine. Lincoln makes a nice one for around $100.
7. Sawsall with some metal cutting blades.
8. Some hose clamps to hold it together while you move the pipes around.
This post is not meant to be a how to thing as much as its meant to give you some ideas. If you have some specific questions then feel free to Pm me or ask here!
GL!