Dave, that setup could be fun for riding around the town and some hills, but it's not gonna work right for serious downhill. The Urban Assault is wedged at both ends and has a fairly short wheelbase - this makes it too turny for high speeds. And Seismics...I dunno how they go at high speed but I doubt it would be good. The Flywheels don't have the best grip but they'll work OK as long as they fit whatever deck you get to replace the UA (they might be too tall). Bearings are fine.
If you're new to all this you've done pretty good in my books.
180mm wide trucks, wise choice.
Each seismics type are designed to suit a specific riding style.
If they're 45deg, they're made for carving.
If they're 30 deg. They're made to suit speed/stability.
If you have one of each you're very lucky. This is the most versatile combination to suit all longboard needs.
30deg rear. 45deg front
Seismics can handle any speed you throw at them. It boils down to ability to ride them first. I've bombed a set of 130mm seismics over 65km/h down conrod and with the exact same setup pumped through 7ft cones.
The stable 30deg trucks come with suitable springs for moderate DH speeds. The 45deg ones come with springs made to really turn. If you've got 45deg on the rear you're gonna have trouble with speeds over 50km/h.
Trouble is they're a very chattery feel. They are'nt a very smooth ride on less than perfect surfaces but still very rideable.
Normal bushing style trucks absorb the vibration and bumps better and are much easier to adjust.
Whatever you do, DO NOT CHOP DOWN THE URBAN. It's a collectors dude. You must have paid heaps for it and when you chop it down, you gonna chop a huge amount of value off. It'll just become a mutant board that no-one else wants, and one that only fits part of what you want.
Unless you're ready to crack it over 60km/h it's fine for your longboarding needs. Rear foot does not sit on kicktail in a bomb! Over to slightly infront of rear truck.
Flys are fine for downhill skateboarding too. I rode the 90mm ones for years. They're just an older design. They're great even for bombing hills in and around your local but can be heavy. They break out easy for drifting and are smooth breaking back into traction compared to sq. lip wheels.
DON'T EVER THROW OUT YOU BONES CERAMICS!!!
When they wear out (the ceramic balls never do) replace the metal inner and outer rings with ones from a set of bones reds (for about $25) and drop the ceramic balls in with the crown shaped retainer from the ceramic set. You'll have a new set of bones ceramics again. Just make sure you keep em clean.