hardened valve seats

Generally how much to have them installed?

How long would valves/seats last without hardened valves seats with a mild cam?

Can I just swap heads at a later date?

(596's {my first porting attempt} milled .040, will go with iron 4 bbl intake and A/C with a Crane 693901 camshaft (same as 318willrun's crate 360 cam/build) I bought these from member: 540slant6 in May 2016.