The splice looks really good.
I misread what you said about the frames. I understand now...pics really do help!
ATV Riders Forum - gusset kit
this thread shows a custom gusset set. TPRusa also sells a full frame gusset set for the blaster if you can stand the spider web theme.
The gusset plates are $89 but the "installation kit" is an additional $350. I'm not even sure what an installation kit is... maybe it's the instructions, who knows. LOL
You can make your own though, 1/8" plate steel and a careful hand can cut out a whole lot of gussets. My local Ace Hardware has a wide selection of steelworks products and while they're not very economical if you need much at all, they are very convenient for a very small project. They carry 1/8" x 3" x 36" plate. It's about $15 for the plate but it would be enough to do at least one whole quad in gussets. A port-a-band (or saw-z-all with a metal cutting blade) and an angle grinder can make some pretty gussets if you have the patience for it.