What burden resistor value to use for current transformer?
Firstly, it may be helpful to remember that voltage transformers and current transformers are complimentary. 


Voltage transformers are happy when open-circuit and have a minimum load resistance value (which causes the maximum rated current to flow).
Current transformers are happy when short-circuited and have a maximum load resistance value (which causes the maximum rated voltage to be generated).


You need to watch a couple of things:


Your CT will have a maximum "burden" that it can drive. That's the maximum the resistance mentioned above.
The CT will have a primary rating - the maximum number of ampere-turns it can accept on the primary.
You need to remember that 8 A is 8 A peak and not 8 A RMS which will result in \$ 8 \sqrt 2 \ \$ A.



  If say im (sic) measuring 8 A max, then with a 1:1000 CT, thats (sic) 8 mA on the secondary. 


Correct - but note whether it's peak or RMS.


  If I want the voltage to be 3.5 V full scale, then R = V/I = 3.5/8mA = 438 ohms?


Correct - again subject to peak or RMS.

You probably want to calculate true RMS current so you have a bit of work to do on your sampling and coding.