How to set FILE #, while preparing SETLFS.

You ask what ‘file# to set’
The value for a “logical file nr” is just a handle
It is effectively a table-entry. A spot to store things related to it.
The value you choose for it isn’t too relevant to the rest of your description.
Are trying to mimic a basic merge command?

Okay..i see the kernal message for the load. After the load, there is no added listing. And! (NTSC 128, Jiffy DOS, 1581) and load commands works, but no listing is shown. this is 100% outside of
this prg. i mean... if i issue MY sys4864 i see "loading filename", and the only listing that's there is my test bed for the routine. prompt says READY. A "/filename" is ran, it seems to load, but there's
nothing other than the test bed listing.. Jeez.. sorry to sound desperate, I've logged all day into this.

c128old:
I load basic code from line 0 to 59999. Which stays in memory the entire duration the prg runs. then, I Link modules onto the tail end of that independent main body of code. Ive been doing all loads in basic, and want to sum-up the basic loading into Machine Code. no matter how i approach this task, i can see the basic interpreter send "loading filename" but once I'm returned to basic, the only code in memory is 0-59999. All linked progs beginat line 60000.. Grrrr..

I've not tried the code out. Here's my two cents' worth:
- load your code into the z64k simulator
- set a breakpoint in the monitor on 1300 and step to see if happens what you want to happen

According to the C= docs, you use a secondary address of "0" to load data absolute.
You are using a secondary address of #15, which is the command channel.
If I were a 15x1 disk-drive and would be asked over the command channel to load a file I would not be able to do so. That is, the filename is likely (hopefully) not a valid Commodore-Dos command.
The string you are sending along to your drive is a filename and you want to load it, so I expect you would use a secondary address of either 0 or 1.
In this case I would suggest you use the
LDY #0 ; secondary address (0=relocatable, 1=absolute)
And this should at least get you the data to the locations put into 'lob' and 'hib' with your poke commands.
(poke3070,(a and 255):poke3671,a/256 The 3070 I expect is a typo not in your code)
After that, since the basic-text does not stop with a 0 anymore (that is why you use the -2 in your peek) you expect basic to see the extra text.

Perhaps you can 'simply' increase the endofbasic text pointer as well, but maybe only needed if anything direct mode happens afterwards.

It appears you find a$ in your code. Any reason not to use the basic command POINTER(A$) and poke that into your NAMLO and NAMHI?
Also, you can use the sys-arguments for this: sys 4864, pointer(a$)and255, pointer(a$)/256 to get the lo and hi in A and X.
Or use 'USR(POINTER(A$))' and build some hate for the basic floating point accumulator ;-)


Finally, there are several hints on how to 'merge' basic programs.
One claims it is as simple as:
BLOAD "[filename]",B0,P(PEEK(4624)+256*PEEK(4625)-2):DELETE 2-1
I'm not sure why a 'delete 2-1' is needed. Maybe this has the effect of setting the basic-end-ptr?