I fettled (fiddled) the falt slide carbs before I went to the dyno, it only took about 45 minutes or so to get them dialed in the rest of the way. Most of the issue was because on my A12 the 39mm Keihin carbs are borderline to big for the motor. With the new cam the performance is much better across the range.......................dyno time was $100

This brings up a point about SU style carbs I.E. CV carbs. The Hitachi carbs if they are in halfway decent shape, can be tuned with out swapping parts, as noted they are made for the A series................I wouldn't use real SU carbs personally

Now for some other fun to throw in the mix, Mikuni makes an HSR series flatslide carb which is intended has a replacement for SU carbs. They are a non CV carb so again somewhat less forgiving to being off tune.

The other consideration is using motorcycle CV carbs...................I only used them as a test fit but I did have a set of Yamaha FZR1000 carbs (CV type) on my engine just to see if they would work...........never drove with them but ran the motor..................one could use a set of CV carbs off a big tandem twin...........TDM 850 or old KZ750 twin etc.

Regardless of what you use, anything outside of what was fitted to a similar displacement and state of tune engine will need some fettling...................you can get the motor running decently and then if need be get an expert to final tune them to save a few bucks.

my.02 of course............Tom