7i80-DB + 7I76 - PWM Spindle ?

I'm using 7i80db_16_7i76x3d.bit

Would I change
"num_encoders=1 num_pwmgens=0 num_stepgens=5 sserial_port_0=00xxxx"
to
"num_encoders=1 num_pwmgens=1 num_stepgens=5 sserial_port_0=00xxxx"

Thanks

Probably:
"num_encoders=1 num_pwmgens=0 num_stepgens=5 sserial_port_0=00xxxx"
to
"num_encoders=1 num_pwmgens=1 num_stepgens=4 sserial_port_0=00xxxx"

(assuming you are only using the stepgens on the first 7I76)

So this changes the step4 on TM3 to be a PWM interface ? What should the config look like in the HAL ?

No, you need different firmware for that.

Hardware like stepgens and pwmgens needs to be compiled into the firmware
so you would need a bitfile that has the PWMGen built in

Once its present you can enable 0 to all available instances of the hardware
modules in the hal file

So what is the best way of getting a PWM output ?

If slow PWM is OK (say 20 Hz), you could us software PWM, as pl7i92 suggested
and direct it to a field output or a unused stepgen output.

If you need faster (hardware ) PWM you would need a new 7I80DB-16 bitfile that
includes a PWMgen, probably on the last 7I76 step/dir pin set

I did find a manual for the BD1100 but its in Chinese and
doesnt seem to have much in the way of interface details
(if google translate did a decent job)