Hughes Engines has OS valves that drop into a slant six, same length and stem diameter, however they are pricy.
Many folks go with other Chevy or Ford valves, as they will have thinner stems and the valves are cheaper. But then one needs new locks and retainers. Longer valve stems can lead to valve train geometry issues. Those are fixable but that is also more money.
The stock sized valves with a good multi angle valve seat job can work for you depending upon the performance goals for the motor.
IIRC you are planning a low cost turbo build using a stock cam. If that is correct, I would stay with the stock valves, get a good multi angle valve seat job, a bowl clean up and the back cut that you have mentioned.
I would definitely put in new guides and with a turbo I would install positive valve seals. Your machine shop will need to trim the od of the valve guides to get positive valve seals to fit.
