Brass can be a bit stronger if large fillets are used to distribute the stresses and the base material is not overheated. The end result can look very professional if you spend a lot of time filing and polishing. Welding creates a heat affected zone where the base metal is weakened by being overheated. This applies mostly to CroMolly, not mild steel. Sheet steel is easy to bend into simple brackets and guides that can be screwed to your plywood base.
Hose clamps work nicely in some applications.
A threaded rod with jam nuts and washers works well in place of the rear wheel axle to hold the rear dropouts in allignment. Bolt this to your base somehow as well. Don't worry if the wood jig burns a little. You'll only use it a few times. Otherwise you end up with too much weight on the front wheel.