340 Rebuild "Musts"

Here's a simple list:

Sonic checking - Must if you are boring it out. An excellent way to waste thousands of dollars is to bore it out without checking and end up with a cracked block. Yes a .020 is almost never going to hurt it. But better safe than sorry.
Align boring - Always a must. You want things to "line up" don't you? Align boring is a basic of "blueprinting" a block. It will line up center lines to the bore. Makes sure everything runs well and within spec.
Decking - This is up there with Align bore. Decking along with align bore will make the engine "square". Meaning everything will be equal to everything else. Bores will be lined up to cam/crank center lines. The deck line will be square to the crank line, which is also very important. Or else you end up with an engine where a piston in cylinder 7 might be .004 out of the hole and on cylinder 1 it's .004 in the hole. Not good.
Bushings - Just replace them. They are cheap. And fresh ones offer good insurance against anything else that might go wrong. A lot better to wear a new bushing/bearing then it is to throw a rod.
Aftermarket rod bolts - Once again, just cheap insurance. Rod bolts stretch. I wouldn't re-use them.

Any of these should really be done on ANY rebuild. Yes it costs money. But how often do you want to rebuild the thing? And as I keep saying with engine building. The devil is in the details. A properly balanced and bluprinted engine, with a strong hone, good decking and excellent rings. Can be worth an EASY 50hp over a thrown together engine without attention to the detail.

Only thing I don't think is ABSOLUTELY needed. Is new rods. If it is a 71. It should have forged rods. As long as they spec out. Give them a shot peen and polish. Then throw some ARP studs in them and back in the engine they go.

Now if you want an idea of 400 horse. My setup was rated (probably under rated) at 380 horse with the 340 manifolds. Now I have headers and a much better exhaust on it. I figure it's about 420 horse.

340 block, with all the work I mentioned above. As well as checking oil galleys, clearences, etc etc. Basically a full balance/blue printing. These details make a difference.
Stock forged crank/rods
.040 overbore
Keith Black 243 hypereutectic pistons
J heads, 1.88 intake valve opened to 2.02. Stock 1.60 exhaust valve. Excellent valve job (3 angle with back cut if I remember, Ill have to check the build notes). And while they did the valve job they also did light cleanup work on the turn, port and bowl.
Comp XE274H with comps recommended kit (lifters, springs, locks, timing gears/chain, etc) comp pushrods
RPM Air gap
HV oil pump
Stock 340 A body pan
Rebuilt/recurved stock style electronic distributor
Rev-n-ator ignition box

Right now. I have a garbage 12 year old 670 street avenger on there. It's some cheap chinese crap. The metal is flaking bad and can't be fixed. I also think I have some distributor problems. I am going to switch over to the distributor recommended by rev-n-ator. I am also changing over to FItech 600hp power adder setup. First I'll just run the fuel side of it. Let it learn, get a tune, see how it does. Eventually I will use it to run timing and nitrous as well. I've heard A LOT of great things about the FItech setup. Mike at MRL has used them on his last few builds and says the thing runs excellent and is worth a few hp over the usual carbed setups.

But as is right now. With 245's out back. A 4.10 gear set. A 2400 B&M off the shelf torque convertor (which I'm replacing with a custom built PTC unit as well). The thing will ROAST the tires at will. Anything over 1/4 throttle in 1st/2nd will light them up on completely dry pavement. If I mat it in drive. It will burn the tires all the way into 3rd easy. I can be cruising at 50k in second and if I push that throttle it'll light them up in the middle of the road. So it's definitely making good steam.