@DEVLINCC is correct about the TT-DT spacing being stronger. The physics supports it.
That downtube, combined with your gussets, should be strong enough. Contingent about weight and riding style…
Gusset Theory
During my trip to Taiwan, I learned more about guests and headtubes. I still need to do my own testing to confirm, but the theory checks out:
- Gussets on production bikes are to pass the fatigue test
- Without a gusset, toe of the weld (heat affected zone) cracks under fatigue
- Ivan did a great test in this thread: Welded joint failures and metallurgy
This info was for XC and trail-style hardtails. Enduro bikes will probably need more reinforcement. However, the theory still applies
What does that mean?
- fatigue is the more common mode of failure
- check for cracks over time
- welds are the weak link
- gussets spread the forces over more weld
- good welds are the key to fatigue life
To give you more confidence in your work: given how steel fails, if the front end feels stiff enough, it’s probably strong enough.
Under normal riding, you are safely in the elastic zone. If you case a jump or slam into a stump, that is a different story. But you could argue that is pilot error not builder error.