I have a 74 RR running on a non original 318.

Original small block to the car is long gone and this is a junk yard late 70s 318 running on most cylinders and some compression. It is not worth a rebuild.

I was looking at a low cost engine options.

An LA 360 or 408 stroker.

A buddy said the best buy he has seen in to get a standard rotation marine 360.

That should leave me able to hook up to the original 904 but would need to change the balance.

Nope he said, they make internally balanced 360s that bolt right in. Just throw all my same parts onto it.

Has anyone done this. It would be great to spend just two K on an engine.

I've never heard of an internally balanced 360 from the factory...then again I've never messed with a marine engine either.

If it wasn't in fact internally balanced the worst you would be looking at is a new convertor...maybe a flex plate...

This is the only one I found on a search.

http://www.summitracing.com/parts/fmm-cr360la/overview/make/chrysler

Looks like you have to re-use all of your accessories.
Without all those extra parts it sounds a little high to me. I'd think you could freshen what you have for less than that.
Marine cam...wonder what the specs on that would be...

High RPM HP and crappy low end torque.

I know absolute zero about boat motors, but a friend bought the twin 413s out of a burnt boat, (never used them, one counter rotated tho) and it seems to me that they had a different rear flange, maybe like an industrial engine? I haven't seen them in years, but he probably still has them somewhere, never parts with anything ...

I suppose it is possible that it was just an old boat, and they were pre 62 style cranks.

It may be that newer boat engines use the standard flange though.

Just something for you to be sure of before buying.