I've tried to tidy up some porting notes into proper documentation, but 
can't get my mind around it, so I'll write it up as a commentary.

If you're building a homebrew project, messing about with BASIC is going in 
the wrong direction. If you're building a home-brew system, the correct 
thing is not to port BASIC, but to port the MOS API, and simply drop a pre-
existing BASIC binary straight into the memory map.

The bare minimum your MOS needs to do is:
BRKV, WRCHV:    vectors for BRK and WRCH
BRK:            point &FD/E to error message, jump via BRKV
OSASCI, OSNEWL,
OSWRCH, WRCHV:  print characters
OSWORD 0:       read input line
OSBYTE &83/&84: read bottom of memory/top of memory
&0000-&005F:    zero page workspace for BASIC
&0400-&07FF:    fixed workspace for BASIC
On startup:     initialise, and enter BASIC at its entry point with A=1

All other API calls can (and must) simply return unchanged until and if you 
choose to implement them.

As an example, the Mini65 MOS squeezes a basic text I/O MOS into 512 bytes.
MiniMOS text I/O using 6850 ACIA:
  mdfs.net/Software/Tube/6502/Mini65
MiniMOS source:
  mdfs.net/Software/Tube/6502/Mini65.src

The SerialIO MOS squeezes a basic I/O MOS into 2K at &F800-&FFFF that can be 
used as a simple text I/O or implement filing functions with a TubeHost 
program at the other end of the serial link.

6502 Serial MOS using 6850 ACIA:
  mdfs.net/Software/Tube/6502/Serial65
6502 Serial MOS source:
  mdfs.net/Software/Tube/6502/Serial65.src
TubeHost for various platforms:
  mdfs.net/Software/Tube/Serial

An example of using these is the un-modified BASIC 2 with a MOS for the 6502 
BenEater homebrew computer:

BASIC 2 with MiniMOS text I/O using BenEater 6551 ACIA:
  mdfs.net/Software/Tube/6502/BenEater/BenBasic2A
BASIC 2 with Serial MOS using BenEater 6551 ACIA:
  mdfs.net/Software/Tube/6502/BenEater/BenBasic2B

Additionally, there's an experimental build I've been working on over time 
on and off:
  mdfs.net/Software/Tube/6502/BenEater/BenBasicC0

This is experiemental development to see if I can strip out a small amount 
of functionality from BASIC to squeeze it into memory at &C000 and leave 
enough memory at &FD00 upwards for a tiny MOS, to allow the whole build to 
fit into a single 16K ROM. This test build strips out AUTO, RENUMBER, DELETE 
and ENVELOPE to allow enough space for the BenEater text I/O MOS to squeeze 
in.

So, I'd strongly advise you to not fiddle with BASIC, but just use the 
unchanged BASIC2 or BASIC3 binary in memory at &8000-&BFFF or the HiBASIC2 
or HiBASIC3 binary in memory at &B800-&F7FF, and concentrate on implementing 
the MOS API. Start with something like the Serial65 client or the Mini65 
client.

See here for binaries of BASIC:
  mdfs.net/Software/BBCBasic/BBC/Port.htm

See here for source and binaries for 6502 client MOSes:
  mdfs.net/Software/Tube/Serial/

All the best.