A very good L6 engine is the Chevy 250 ci (also available in 292 ci, taller block). Excellent engine, nearly as smooth as a V8, dead reliable, common because produced over many years. If you are not looking for high performance, even the "pollution-controlled" engines from the '70s would work well. Often, they are available for sale because people get rid of them to mount a V8 instead. Even here in France you sometimes see one in the classified!
You can use the same trans as behind a V8, including a 4-speed auto (TH700R4), a really good set-up that allows modern highway speeds.
As for the specifics of installing one in a '49 truck, I do not know.
The roddoing scene in the UK is very strong, try to see if that community (via UK forums) can help you find what you need.
