Apologies in advance that this doesn't directly answer your question, however it is a solution to your current problem and will allow you to progress with your project, and provides a good opportunity to post useful info about joint strength.
I would like to use this plane to strength glue joints for the side of a toolbox.
This is not necessary for strength*. Butt-jointed long-grain edges already create a bond stronger than the surrounding wood (assuming they are well formed and glued together properly, with sufficient adhesive and clamp pressure).
Tests have confirmed repeatedly that if you glue boards together, wait for the glue to fully cure and then break them apart the glue joint holds and it is the wood itself that breaks.
[Source: Fine Woodworking excerpt.]
So you could solve your current issue by not using the Stanley no. 48 and just using a regular plane to joint your edges flat.
*Despite how counter-intuitive this seems, when joining boards along the grain, tongues and grooves, biscuits or splines (floating or glued) do not contribute significantly to joint strength, they act primarily as alignment aids.
Even pocket screws spaced along a long-grain joint do not add needed strength, they would primarily function as clamping devices in reality. The screws can in fact be removed after the glue has cured if desired, without resulting in a weak joint.