Your choice should be based on what you want to do years from now, because one day you will wake up and realize any mistakes in choices you made a long time ago. Budget will be a huge part of it. If you want to build power over 750 hp I would highly recomend an aluminum block with a big bore. A 4.440 bore AL block should last a lifetime for most. Many rebuilds. In a duster at stock location you end up with a front / rear weight distribution good for nine sec ETs.
If you think you will want to go faster as time passes then a 540 to 560 cube aluminum shortblock makes a good base. I would go with heads similar to 440-1, since you can have pistons made that will work with many different heads allowing head upgrades down the road. A short dome for future compression increases, and valve notchs cut for the biggest valves and cam you will ever run. You can start with unported heads to help budget for the AL block which will set you back about $6800 ready to assemble. This all will give you a base to reach 900 hp at some point if you want. Time flys. I built my first nine second motor in 1998. I made two big mistakes. I sold a billet 4.5 crank, and didn't go with the AL block like I should have. Those choices eventually cost me a lot of money.

Lots of interesting answers....I guess the real answer is, it depends on budget, and ultimately, the weight of the car with driver. Me, if its over 3000 w/driver, I would probably not do a stroked small block where you probably have to buy an after market block and heads. The cost could easily exceed that of a stroked 440, which based on the number of 440 combo's easily going that fast at 3200lbs, is one of the best combo's for the money. Even on pump gas, 750hp is attainable with off the shelf parts and a stock block.

The big Hemi, if you do not have a block already, options are limited and pricey and may have long lead times for the block anyways. I like them, but they come with their own set of issues. They will easily take a big heavy car well into the 9's, but you can spend a ton of money making one of them work if you can get a block.

Now a NA G3 Hemi will run in the upper 9.80's 9.90's in a 3200lb car with driver, and relatively cost about the same or a little more than a 440 stroker, but with less maintenance in the long run.

A Hellcat engine will be the fastest of all for the money....If you get one from MMX for $15k and the efi from FastMan Rich....Those things run...We are talking 9.10 at that weight....And its dependable!

The LS platform, well, there is that....I am not putting one of those into my Cuda.

That is the most vulgar and gross thing posted in the history of Moparts.

If a Hellcat has the same low gear as a Demon, 4.71, it's going to take a better man than me to get it to launch properly. A mans got to know his limitations.

Really depends on your budget. The larger cubic inch the less rpm you need to turn it. A 572 with good ported 440-1s would make plenty of power and last round after round at 800 hp.

I've been in love with the 426 Hemi for 49 years so it would be an easy choice for me.

Small block geek here... I’m blind to all others

RPM breaks more things than power.
In 10 years everything will be boosted.

A 9 second A body aint exactly rocket science. At least I dont think so, but one trip to most any racetrack makes me wonder how so many 500" plus BB Mopar strokers struggle to run low 10's We went 9.60's with an iron headed all steel 474" BB in a Duster 20 years ago

Weight of the car makes a big difference. Is your car a 2400lb car or a 3200lb car? If it were me and I was using a stock block, I'd go with a 400 stroker. A 440 stroker will get the job done as well, but it seems the 400 block is a little bit stronger. Should be able to buy off the shelf parts to build a BB to get an A body into the 9's (atleast high 9's). You could get a small block there to, but will require more work and maintenance IMO.

Too bad all that exotic "truck engine" stuff you have, isn't made anymore... It works, but getting hard to find a good block, and heads. Out there, but I know my buddies W8 combo was a machinists nightmare.

There is a lot of ways to go 9s in a Duster, but you asked for the best. Hands down, 572 pump gas Hemi. Red line it at 6800 and it will live forever, cheap to operate, huge resale value compared to anything else. And the whole range of 9's is available, from 9.000 on up. There really is not reason NOT to go with a Hemi.

Well cost is one of them... Depends on what the budget is. Most parts cost the same, but valvetrain, heads, intakes, all cost more. Then there is the fitment issue of a Gen2 HEMI in a A body. Not denying the cool factor, but there are SOME drawbacks.

Heads cost about the same, the only thing more costly on a Hemi vs. wedge is the block and the rockers.

But look at the cost of ownership, factoring in resale. How many used Gen 2 hemis do you see for under $10,000? How many used wedges for over $6,000? You get the difference in up front cost back when you sell.

Couldnt agree more. See more 500 inch bog blocks that either struggle to get out of the 10’s,or barely do. Even ported big block eddie heads flow plenty of air to run 9’s unless the car is ultra heavy.
Guys with 440-1 heads on strokers in gutted race cars running high 9’s cracks me up. See it constantly at the track.

A 416 with Indy heads in a 300-3200 pound A body will dip well into the 9’s shifting it 6800-7000 grand too.
Had a similar to the above W5 deal i shifted at 7000 that i had hundreds and hundreds of passes on it at 3350 pounds that it took a very hot day for it to run 10 flat, beforei sold the car.
Had 1/2 the money it it a hemi would cost, and not the extra weight on the nose

Ok im going to be that guy that everyone hates right now.
You sgould be able get a complete 6.4HEMI for about the same money as that indy topend will cost you,and the 6.4hemis heads will walk all over those indy heads,they move stupid amounts of air and the blocks are also plenty strong. rotating asemblies are awailable and not stupid expensive either for all forged internals. if you cant get a 6.4,an eagle 5.7 is a serious foundation for good power aswell.

Then it becomes getting headers and fitment, etc. etc.