I assume you are referring to boiler construction. A 7.7kW propane torch is only suitable for small scale silver soldering. I used such a torch to attempt to build a 3.5" gauge Rob Roy boiler. It failed to provide enough heat to silver solder the smokebox tubeplate and the backhead. An 80kW 50mm propane torch is possibly the largest available and I would obtain that size if possible, although 40kW has been used for reasonably large boilers.
Built 3 1/2" Gauge LBSC's LMS Black 5 Doris and Martin Evans Rob Roy Building Don Young's Black 5