How difficult would this be

first, unless you are planning on aluminum heads, intake, front drive system, etc, those engines weigh a ton.
second, there is the rear mounted distributor to take into consideration, firewall clearance issues.
will you be able to find an oil pan that will fit, or will you have to fab one ?
then the transmission adapter needed for whatever transmission you want to use.
it will be a TON of work and not cheap, but can be done. however, for the dollars spent, you may be better off building a low deck stroker.

of course if you are talking gen 3 hemi, i have no experience or advice.

That is what I thought! I have 2 fresh 383s and and all parts needed to make the Dart drivable.
NO HEMI NO HEMI

Yes, just like a small block !!

The biggest issue will be the width and then headers, etc. Shorties would probably be the best option. Swap the stock front end for aluminum stuff and a BB Chev water pump. Use an aluminum intake and you're reduced the weight significantly down to roughly what a big block would weigh. Certainly no more than a gen II hemi!!.

Sell the 354's to a rod or LSR person. Use the money made to do the 383 or other BB. Maybe even a GEN3 Hemi. Gen 1's are not cost effective.

I saw one for sale a couple of years ago in Montana with a 354 hemi in it. I don't know what it took to do it but, it looked like it was meant to be in there. Don't remember if he had any aluminum on it or what pan he used. I would think it would be nose heavy as h#ll.

Op, I'm sure a stock BB 383 or 440 weighs more than a early 354 C.I. all iron Chrysler fire Power Hemi do. Do some research to make sure though
I've own and used both, the bare B and RB blocks are heavier than the bare 1954 to 1958 hemi blocks are. Not so on the heads but I have never measure the actual weight of a stock early hemi motor with exhaust manifolds, motor mounts and so on.
I don't remember the advertised HP on the 1956 354 C.I. New Yorker and Imperial motors but I'm going to guess around 275 up to maybe 300 HP. The 1955 331 C.I. New Yorker and Imperial motors had 250 HP advertised and the 1957 and 1958 New Yorker and Imperial 392 C.I. motors where advertised as 325 HP is what I'm remembering now.
The "300" cars that had dual quads and solid lifters motors had around 50 HP more than the single four barrel hydraulic lifter motors of the same year had

The weight of those things is always the first negative. Everything on those early motors is cast iron. But so is everything on any other "stock" motor. The only thing making those any heavier than any other motor is the larger heads. And even then, they still don't weigh as much as a gen II hemi. But as soon as you start replacing some of that iron, the weight drops significantly. Timing cover, water pump, intake, distributor ... there goes 100 pounds right there. My point is, don't belittle these motors because of the weight. The only real issue with these motors is the cost to build them and in reality, if you did a similar parts swap on any other Mopar motor the difference in cost wouldn't be that much.

Since the original Eng and Trans are missing I wanted to do something a little different.The old Hemi looks cool but I want to cruise with it.
What are thoughts about running an Edelbrock STR15 Cross Ram set up? I have an intake that has never been on a running engine so I will need
Carbs,Fuel lines and Linkage. What are thoughts about a lift off hood like the M code B bodies. I have thoughts about cruising with the hood off
and the carbs exposed.

any intake can be made to work if enough tinkering is done with it.
as to how WELL it will work, that is another matter.
this question should be asked over on the racing forum. those guys have way more multi-carb experience than i do.
along with carb tuning will be your ignition curve. those two items go hand in hand on how well a combination can, or will run.

There was a Gen 1 Hemi Dart in a mopar magazine a few years back- it was gold and may have been on the cover. It fit. It worked. I think it is a terrible idea- please don't, but at the end of the day, it is your car to enjoy. A 383 GTS is a pretty rare car. Better to cut up a 6 cyl/318 Dart if you want a Gen 1 Hemi Dart. There are a million ways to create some eye candy on an old muscle car. Find a different path! Good Luck.

Rick

Engine
Weight
Chrysler 331 Hemi
700
Chrysler 300 Poly V8
691
Chrysler 354 Poly V8
691 - 700
Chrysler 354 Hemi
720
Chrysler 392 Hemi
740
DeSoto 276-341 Poly V8
675
DeSoto 276-291 Hemi
630
DeSoto 330-345 Hemi
670
Dodge 241-325 Poly V8
645
Dodge 241-270 Hemi
580
Dodge 315-325 Hemi
610
Dodge Viper V10
716
Dodge Viper V10 (1997)
648
Dodge truck V10
816 - 836
Mopar Slant Six
475
Mopar 277-301-318 Poly "A" V8
600+ (estimate)
Mopar 273-318-340 "LA" V8
525
Mopar 360 "LA"
550
Mopar 350-361-383-400 "B" V8
620
Mopar 413-426W-440 "RB" V8
670
Mopar Street Hemi
765

Like driving with a 175 pound guy sitting on the hood.

Reminds me of one of the first model car transplants I built.
1964 Barracuda Fastback I installed a dragster engine in - blower and all.

You will be the only guy on the block with one. And the city, and probably the state.

But that’s what this hobby is all about.