best motor into a needle nose?

Do you have any pictures?

1) i know you said you don't like computers, but really the ddec system is pretty reliable and simple. i don't see why this(a detroit series 60) wouldn't fit.....

2) i don't think it's so much a question of how much hp the radiator can handle but more a question of how many cubic inches it can handle. More cubes = more heat etc.

These 2 points are why i was asking for pictures.

 

There is absolutely no room for an air to air setup.

Stick with a 855, a good 400 big cam tuned right wont take much more to cool than the one coming out of it.

 

You may not like an electronic engine but, if I was you I would call Cummins and ask about an M11 drop in engine they are light you can get all the HP you want, the Radiator it can be re cored, extra rows. the inlet should be good, also the outlet location, the size you can change or get the outlet pipe made up to fit.
With 4.56 gears I take it will be a dump truck.
If you take the truck serial number in they or a KW dealer should be able to tell you what it all had, and Cummins will be able to tell you what it will handle.
The front mount will need to be relocated and the fan will need a spacer, that is the easy stuff.
The rear mounts leave as they are.
I did re-powers as a living for a while, mostly Cats, to Detroit's at that time.
Just a thought!

 

With an M11 you can move the radiator back and there are Rad shops that can make you a CAC to fit.

 

Id say either a big cam cummins or even better a b model 400 cat. The 400 isnt air to air and they are a great motor. My friend put one in a 54 pete and it seems fine although he only pulls a big camper with it.

 

I vote for a Big Cam Cummins. The 855 block is like the old Chevy stovebolt six, tough and easy to maintain.

 

I believe the 290 Cummins is an 855 cubic inche engine as is the 400 hp Cummins engines.