i would say the way to use the kmh as your mph, you would need to go to a trans. shop and have them adjust the speedo gears within the trans. as far as the 305 heads the vavle will probably be so small as to limit your horspower with a 350. I think the heads of choice right now are the new vortec heads from gm. these are not a simple bolt on, but from what i have heard they are very impressive. ideal valve size would be 2.02intake/1.60 or 1.65 exhaust. 292 GM castings are nice but they are getting pretty rare. their are a lot of aftermarket heads on the market. I guess it all depends on how much money you want to spend.
have fun and good luck.