Re: steel tubing for frame?
unless u plan on using variable thickness steel. perhaps local lengths of tubing would be more than enough. 1/16 1/8 etc thicknesses is what you need.
dont worry about grade of steel. if it truely looks great when ur done the mild steel w be cheap and ride great possibly.
3lbs light frames are not stiff so why you need good steel if u makin a beauty 6-7 lbs frame for 40 bucks or less. the triangles will be fine.
there lots available off the shelf like universal droppouts etc. but 69.99 handbuilt raw cruiser frames are available allready. maybe this helps. whats the plan?