I know that a 3/4in burner is good for about 350cui but how many btus are need to reach welding temp? 

No way to tell. BTU is only one of a number of dimensions in a welding temp forge and one I couldn't tell you regardless. What I can say is a well tuned 3/4" propane burner will bring between 300-350 cu/in to welding temp reliably. There are other things that effect this though, how open are the doors, so open it's loosing too much heat or so closed the burner can't operate efficiently? How well insulated is it? 

How about the factors that don't directly involve the forge? What kind of steel is it? Is it clean enough, and the faces matched properly? Using an appropriate flux?

You can reliably weld at orange heat if the faces are well matched, cleaned shiny, properly fluxed, closed tight and allowed to come to temp throughout. 

Have you given Tristan's "Alaska Flux" a try? Last time I did it had a bit too much charcoal in it but it works a treat. I always have a can of Peterson's Blue welding flux in my tool bag at meetings. Give me a shout and I'll show you how well an inexpensive commercial welding flux works. It's about $26.00 a pint can at Weld Air or Aire Liquid shipping included. The stuff works really well. Coming to the Jan21 meeting? Remind me and Tristan may be able to make it and bring some of his latest batch. Maybe we can do a mini workshop 

Frosty The Lucky.

I'm going to be up there the 21st. 

I've been really intrigued by the narb you built. But I don't think I'm going to beable to build on for my self so I'm thinking about buying one of  Dudley Gibberson's but all of his are in btus so I'm not sure how big I would need.