At this point I don't know what machine you are thinking about converting, aldot. Can we have pictures of the machine, the original (Briggs) blade setup, and the blade boss you are thinking of converting please?
I made a conversion a couple of weeks ago, and it took a lot of extra work because I had to do it twice due to insufficient measurement work before I started. (I wanted to put a Honda HRU blade plate with 4 swing-back blades on my 1980s 4 height lever bar blade HR. I also wanted to lower the cutting edges of the blades 10 mm below where the bar blade's cutting edges were, so they were at the bottom of the mower base.) On the first try I somehow managed to overlook the fact that the Honda blade disk is heavily dished downward, so I put in an hour or so of milling machine time making a complicated 5" diameter 15 mm thick spacer I didn't need, to adapt the huge bar blade boss to the small pitch circle required by the blade plate. And yes, I could have just bought an HRU blade boss instead of making anything, but I'm way too cheap to do that.
In reality I just needed to drill and tap three holes near the middle of the original bar blade boss, and make a 1.5 mm thick spacer to keep the blade plate from fouling on the original mounting spigots for the bar blade. The conversion is therefore instantly reversible: if I ever decided to revert the old HR194 to original form, it would only take a couple of minutes to do it. And yes, it works extremely well so far. The point was that it's easy to miss one detail of the physical layout when you are doing the design, and end up going off at a tangent.
I have the impression that Honda's output shafts (PTOs) are longer than Briggs ones, to accommodate the blade clutch they tend to use. You will therefore need to pay careful attention to where the cutting edges of the blades end up relative to the mower skirt and bottom of the grass scroll. You don't want the blades hanging in mid-air 10 or 15 mm below the bottom of the mower, like an old Victa toe-cutter. And you don't want them 10 mm above the bottom of the mower either, like the 1980s HR194, or you won't be able to mow down to the 30 mm cutting height commonly used for kikuyu without the mower skirt burying itself in the grass so you need three guys to push the mower.
