Pictures? It's usually a good idea to get that figured out before you make the engine mounts, so you can juggle things as needed to make it all fit. Have you looked at manifolds or headers for a 340 in a 1970ish Dart?
It appears/I believe member "mcmopar" has a 37 Mopar P-up with a 340 in it, he may have an answer for ya. https://www.jalopyjournal.com/forum/threads/340-in-a-1937-dodge-truck.1192170/page-2#post-14099163
I had a 37 Dodge pickup that I put a small block Mopar in. I built my own headers but that's not to say you couldn't find an existing set you could modify to fit.
I have a set of manifolds from the 69 block but drivers side gets into the the side skirt of the hood
340 same as any small block maybe set of block huggers or any short style I have stock manifolds but used these mounts if it helps. In 33 Plymouth should be about same space
The type of steering you're using will make a difference in how things fit...so pictures of your setup will really help folks help you. We can only guess at what you are looking at.
I put a 318 into a '41 Fargo and used the left side manifold from a '70 or so Duster. It curved up and around the steering column.
Have you looked at these? https://www.speedwaymotors.com/shop/340-chrysler-small-block-v8-headers~131598-75-9-11-524-31037 The block hugger type are used on lots of rods.
There are couple of different styles for the Dakota that might be useful. dodge dakota headers - Bing images
Amazon.com: Flowtech 11504FLT Black Standard Headers : Automotive Have these on my 32 Desoto with a 360.
I am working on a second set of stainless one's for my truck now for my truck now. I ran my exhaust between the fender/running board and frame. It worked out pretty good for steering. I also had brakes and clutch in the way on driver side, with a battery on passenger side, thus the reason for outside the frame. It took a while to build them as I am no professional, but i did it the work great and the truck runs hard. These are the old ones.
Yes it’s very tight looking at block hugger headers at Summit, they dump in the centre might be alright
https://www.summitracing.com/parts/...1939832705289&utm_content=GSAPI+5ba28fd9454f1 Whatever you chose may need some modification, but a lot easier that starting with flanges and a pile of tube bends.
Equal length and clocked for firing order. I know its over kill but makes me happy. Looking fo a 10 sec run this year.
Patriot makes a shorty header with center dump that I used on my 41 Plymouth. It would not have worked with the stock steering gear, however.
Don't know how close the engine compartment on a 37 pickup is to a 37 Plymouth car, but these are what I have in my 37 coupe. Custom made long tube headers.
I used off the shelf headers for '72-'93 full size 1/2 ton Dodge pickup 2 wheel drive , they fit inside the orginal frame on my '38 Plymouth pickup . Didn't have to dent any tubes , fit like they were made for it. I believe the brand i used were Hedman .