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A while ago, I was looking into using a 298 mm disc with the stock RD350 caliper and a RD400 slider (to get the caliper behind the fork) What I found was that a caliper adapter would be quite large and triangular in shape, so I shelved that idea for a while.
That 298 mm disc was an XS650 disc hub that I refurbished, with a custom ordered disc from ISR in Sweden. Sort of a "sleeper setup", but the necessary caliper adapter would stand out too much for my taste.

What about TZ fork or sliders?

Also, the 38 mm forks from FZR400 and FZ600 have the same C-C measurement on the caliper mounts. But I am not sure what size brake disc those forks, combined with an RD caliper would accommodate. Also, I do not know what offset requirement this would result in.

Another alternative is fork sliders from a 1976 XS650C. US market 73-76 XS650 had  a single 298 mm disc, and the same 2x48 mm caliper as the RDs. These 73-76 fork sliders are a little longer than the RD sliders, due to the 19" front wheel, and also a bit thicker and heavier,  but can surely be machined down for more appropriate dimensions for an RD. These XS models had 34 mm fork tubes, like the RDs, but only the 76 had the caliper behind the fork slider, just like the RD400. This way, no caliper adapter is required. I have also seen other custom bikes where suitable caliper mounts have been welded on to fork sliders.

the top out springs aren't in the 400 forks... but they are in the xs650 ones. just as a point of interest.

Mark, yes the later years of XS650 had top out springs, but my 77 XS650D does not. It is really a bit weird that the rather wheelie prone RDs did not have them :)

There are several possible contributing causes to this "top out clunk" phenomenon:

- Too much spring preload and/or too stiff spring rate
- Not enough rebound damping (Oil too thin, or someone may have modified or messed with the damping rods/ valving)
  There should normally be significantly more damping on the rebound than in compression. Just remove the spring, add
  a little more oil, and test by hand.
- Excessive unsprung weight. Like a heavy aftermarket fender, a second brake disc and caliper.I had this "clunking"
  during accelleration on my then totally stock "Euro" XS650, with extremely heavy OEM dual front discs and a heavy
  OEM chrome front fender. After upgrading to a much lighter brake setup, etc, I lost around 10 lbs of unsprung
  weight, and the "clunk" was gone.
- Some bike also have a "top out spring" that will give the forks a "soft stop" as they extend fully. These are often
  20-30 mm long, and sit on the outside of the damper rods. Just google "top out springs".

definitely iron barrels, teazer. after talking to uncle ron at fahron this morning i asked him about squish, as obviously the 550 doesn't have squish bands. he reckoned it was an easy fix, just machine the heads out to accommodate the oversized pistons, then reprofile the combustion chambers to get the compression ratio right. mine of information that old lad is 
cheers, gil.
p.s. ron isn't doing bad, he still works 6 days a week and he was 78 last month. hope i last that long 

Gil, when you start that project, it would be interesting to know cylinder spacing (C-C) and cylinder stud spacing.

Yoshimura spring 

well THAT certainly changes things, had no idea that even existed.   

Me neither...Suzuki sure were early with Nicasil plating, based on that info.

Interesting:
https://www.oldjapanesebikes.com/mraxl_GT_Resource/GT550_files/page-9

id go to 62 leave a bit of wiggle room and have at it w the 550 cyls.. but the tranfser ports are dismal. you'd have to add a boost port or 2 and machine the windows in the pistons but would prob be ok. the reeded h2 i had was around 88-89 rwhp. (and extremely rideable).

Yes, the Suzuki 380/550/750 transfer ports were the reason for me wondering about this in the first place. I do not even have a GT550, but I somehow always thought they looked good.

A "Yamahase" 🙂

I just realized that the Suzuki GT550 has 62 mm stroke, just like the RD400. The GT was in a lot more moderate state of tune than for example the H1, with 50 vs 60 hp (manufacturers numbers, of course)

Now, if one could build a GT550 based 600, using 3 x RD 400 cylinders and heads. Then it would theoretically be 64-65 hp (since Yamaha said the RD 400 had 43 hp)

This surely must have been done by someone already? From the pictures I have found, it seems the cylinder spacing on the GT 500 should allow RD400 cylinders to be fitted. Most likely the stud holes would have to be welded shut, and new holes drilled and tapped?

I really like that ding-ding sound of Lomas pipes. I just wish they made something between their Retro pipes and these (too) modern looking ones.
Basically less upkick, internal stinger, and one piece design. Like Spec 2 classics, just a with a larger silencer outer sleeve (60-65 mm od)

Was the T500 really that fast in the quarter mile?