Silver solder braze likes to have a fitted overlap. Butt ended joints do not offer much strength. Silver solder is best when sweated at around .002" to .006" thick. You may be better off with bronze or brass brazing. Very similar but higher temperature. If you don't have an oxy-acetylene outfit, you may borrow mine.
With either silver solder or braze it is important to not overheat the filler metal.
Also, you should consider making a fixture to hold all parts in perfect position during brazing . Very important! You may be able to make most of the fixture from hardwood, we did at work often. In the areas exposed to welding flame, you may have to use metal support/locators. Use your imagination a bit, screw heads of screws extending from the wood base etc,
Build your fixture around the engine itself using it to form and fit the connecting tubes perfectly.
PM me your phone number, easier to discuss that way.