New/Wanting to build

No kit will allow you to cruise at 40 all day without some minor tweaking and a 36 or 40 tooth sprocket.

Other than that, buy one from about any vendor here, my vote goes to thatsdax.

I am still running the stock 44t sprocket, my top speed is around 30-32 mph.

There is a shifter kit available that should allow you to get that type of speed. However, I beg you to consider that these are bicycles and aren't designed for the type stress that those speeds put on the system. A critical frame failure at 40 mph can be extremely painful/fatal! If you want to run with the big dogs get a motorcycle!

I know this doesn't fit your current thoughts, but if I were building again new for the first time, I'd get a cheap track fixie bike on e-bay (under "track bicycles") and they sell for about $230-

You get a complete bike that's minimal- and has a rear flip flop hub that our sprockets fit right onto and lockdown (at least laterally) with a reverse threaded lockring-

Flip/flop wheelsets alone are over 100 dollars-

a minimal bike with two handbrakes, narrow 700 C wheels and lightweight- This thing would really roll! I think I'd go now iwth a smaller 50 cc anyway, and get a 36 sprocket right off, for a better cruise-

I got a Micargi cruiser multi-sped cruiser, and the sprocket fits over this hub too, with no modifications, but doesn't thread or lockdown - so the rubber and heavy metal backplates are needed-

Although with a 36 and smooth 1.75 tires, this 66 can go as fast as I want to go anyway, it's still heavier than I'd like, and I'm itching to try some 700c wheels with a high flange flip flop hub, to bolt a sprocket directly to, and eliminate all the other rubber and plates--

they'd fit the cruiser even, if I put road brakes on- but then again, no.