Comparing 6061 vs 6063 alum.

[cdlwingnut]

I know 6063 t5 isnt as strong as 6061 t6
Is 6063 t5 acceptable or only 6061 t6 for a small slow single seater

[Bob H]

The mechanical properties of 6063 are intended for architectural uses not aircraft. Big difference in tensile and compression strengths as well as fatigue life.
Don't substitute 6063 for 6061 for any structural application.

[Frank Giger]

Don't substitute 6063 for 6061 for any structural application.

This.

If it is structural, stick to the material plans.

If it's cosmetic, one has some leeway.

If you're unsure whether a part is structural or cosmetic, the answer is that it is structural.