I just got my Miller Dynasty 350 yesterday. I'm going to run it with single phase 230V.

Here is the manual:
http://www.millerwelds.com/om/o216869k_mil.pdf
relevant pages are 26 and 31

The cord on the welder is 8AWG.

I'm planning on using a 50A breaker, 8AWG wire (10AWG ground), and a 50A plug/receptical. Does this sound ok?

Can I make an extension cord out of 2x8awg with ground?

Does anyone have access to a NEC book who could tell me what Table 310.16 and Table 400.5(A) say?

Thanks.

Sounds good to me. Miller says that machine requires 47 amps of 240 volt single-phase to run at rated output so your #8 wire/cable should be fine. The 50 amp plugs available at hardware stores and such vary greatly in quality and durability and should be chosen carefully. I HATE those hard black plastic angle plugs they sell with a passion. Every month, you can take them apart and the screws are all loose or stripped. Try to find a rubber-covered plug with a real cable clamp built in. Should cost around $40 if it is any good. Also, avoid aluminum cable if you can. Hope this helps.