You can use that kind of torch successfully with MAPP gas and braze your rack with silver solder 45%. Silver is a lot more expensive than brass, but it does not take much. I found cheap silver on ebay. Be sure it is something like safetysilv and contains no cadmium.
This level of heat from these torches is really not sufficient, and it requires heating tubing to a much higher heat than would be required with oxy supplemented fuels. It is something of a pradox that the cool fuel requires more heat to work, but that is the way it works, with both welding and brazing. The greater heat can burn the flux, so heat control is dicey. However, the end result seems strong, and 4130 can take the high heat which is basically similar to what would happen with brass brazing.
Attached rack was made of 4130 and used a MAPP gas household venturi type torch. Survived a long tour with no signs of wear.
If you are making a rack for paniers helps to customize the rack to the specific paniers, so start with those, then move onto the rack. The other option is to go for a generic rack if you can't decide on only one kind of panier.