No the Vape flywheel is not keyed so tightening the flywheel very tight is crucial, ask me how I know

Correct, the Vape needs timed. Rex's does not as the og keyed flywheel is used and the rex is set to that. No adjustment required. You do have to have the proper FOTO flywheel though. The rex stator plate has only fixed holes located and not slotted. The rex also upgrades to 12V power.
I have already fitted 12v horn and tailight bulb. Just need to get a H5006 headlight.

Rex does claim you can use the key and don't need to time at all

As soon as I get the Rex ignition, I will review it.
On another note, there was no chain on the bike. I ordered a 520-114 link chain. Upon attempting to install, it's way long. The existing sprockets are 15-40. Stock is 13-45. Has anyone ran the smaller rear? If that gearing would be ok, I could just break the chain to the neccessary length.

Not sure where you got those gearing specs?.

14-40 is what Yamaha Literature shows was stock on the 400b.  Several members have gone to a 15 front which reportedly makes the bike smoother to ride (less "surging" at light throttle cruise)

(It was the 250 that was geared lower...  14-47, not 13-45)

 

As always, awesome info here! I got the sprocket sizes from partzilla. I will just break the chain to size! Looks like I already have the 15 counter for the surging.

Be careful with the Yamaha parts lists as they are often shared between models (like 250/400) and it can be very tricky to make sure you're getting the correct part sometimes.

I am also working on a ‘75 DT400 project found locally on Craigslist. Just getting through DMV to generate a title before spending much on it. I have owned some Yamaha off road machines in the past, but my expertise is really British motorcycles…Norton mainly.

Welcome Illf8ed from down in NZ. Look forward to seeing you bike/bikes. Not sure about my expertise but used to ride B31 B33's & even an M20 on the road. & MX a methanol burning B33 in the mid "60s. Man i miss those days--depressed now lol. Oh, & a triumph metisse from "67 on until the Yamy DT1 arrived in "68 [worked for the yamaha dealer]. Now i've gone full circle & want my old BSA back.
Sorry i shouldn't be hogging your post but couldn't stop myself lol once i got back into photobucket--keep making it difficult to get in or just my geriatric computer skills. I'll leave these two pics small so not taking up space--i think. Hit to expand. AJS 350 in early "60's & B33 in about mid "60's.
Sorry, they ended up big.

RT325 love seeing your old photos!! Those were the days