Max that is still a bit vague, so I’ll throw out a few things here that might give you some food for thought. From some of the questions you’ve asked, it seems one of you requirements is that when you open the hood (if you build a car that has one) is you want the WOW factor of having an engine a bit different than anyone else’s. You’re pretty much looking into some of the 50’s -60s nostalgia type stuff that is currently less than common.
In the majority of cases most people start with the car they want and from there decide on the drive train that will allow the vehicle to meet whatever their goals are. In this case I’m under the impression you’re looking to determine the motor first and then are going to decide what body it will end up in. I’m not saying this is the wrong way to go about it, I’ve done the same thing from time to time.
The size/weight of car itself will be the determining factor as to how much motor you will need for your goal of 11-12 second times. For instance a properly prepared flathead in a T Bucket could probably do it but the same engine in a full sized sedan would have a much harder time.
Speaking in generalities the smaller the engine the harder it’s going work to achieve the same speeds/times. My own preference generally goes the bigger displacement engines that require less exotic speed equipment to make the same or better power than a smaller displacement engine.
The Hemi you’re currently considering is definitely a good choice in the WOW factor and power department but over the last few years have become a lot more common place than they have been previously. If you have deep enough pockets to build one of these it’s definitely a safe choice to meeting you’re goals.
I’ll throw out a few more choices that might also fit the bill for you.
While personally I would not necessarily build a 331 Cadillac, I would not be afraid to do a later 365 CI Caddy. While the build would be a bit pricey and probably require some one off pieces (pistons and cam and headers come to mind). They were available with both dual quad and tri-power intakes and would make a great conversation piece and should produce decent power. One down side that comes to mind would possibly be adapting the engine to a more modern transmission.
Buick Nail heads especially in the 401 and 425 versions made very good power right out of the box and vintage speed equipment also occasionally shows up for these.
Ford FE are also good engines at a minimum I personally would not build anything smaller than a 390 and would actually prefer a 428 although I would not throw a 406 or 427 if you’re lucky enough to find one.
Chevrolet W motor (348/409) personally I would go for the 409 in large part due to the displacement and better heads. Unfortunately if your heart is set on a factory Tri Power they only were available on the 348s and port size difference does not allow them to be used with 409 heads.
IMO one of the most overlooked engines are the Chrysler Poly engines. If memory serves me correctly 57 and 58 saw the largest displacement Chrysler Poly (Spitfire) engines at 354 cubic inches. This engine was actually based on the 354 Hemi and the majority of short block parts interchange. Although the heads did not flow quite as well as the HEMI heads, they still did VERY well. Although I’m not positive I believe any of the 354/392 intake and exhaust manifolds will fit with little or no modification. One engine build I have always wanted to try is a set of 354 Spitfire heads on a 392 block. There is just something those old scalloped valve covers with raised SPITFRE lettering that always appealed to me.