Personally I would lean out low-throttle/low-end for more power and speed. AFR of 14.5:1 at 33-50% throttle is OK. On turbo engines, I prefer it around there for hottest mixtures to spool up turbo quicker. On NA engines, @ 75-100% WOT, I like 13.5:1 where max-power is made.
You can braze aluminium together easily with lower-temp soldering rods. Works pretty well and just as good as welding for something like this:
https://alumiweld.com/
Trick is to mark the joint with black soot 1st. Run torche with no oxygen for smoky flame and coat joint with it. Then turn on oxygen and when all soot burns off, joint is at proper temp. Then apply brazing rod.
Actually, make it a 1" crossover pipe. EX500 engine has 360-degree crank vs. 180-degree on 250.