I have a set of guards here and just looked at them, they're a bit longer than the Beetle guards are, but tht might not be a bad thing.
Once the aluminum was cut away on the sides to conform to the 356 front body metal and remove the hollow where the bumper would normally fit, the 356 guards are a little thin in the side profile. The length accentuates this and the dead on front profile begins to look a little bit spindly. A shorter, stockier Beetle guard might be a better choice.
Just saying.
IF you decide on using the 356 guards, you should only trim the sides a small bit and rely of the fit and finish work to carry the day. Trimming and then filling the missing area with welded in aluminum stock will cost you. It demands someone good with tig welding, and someone who is very good with metal finishing to grind the welds flat, fill the pin holes, grind, sand, and polish back up again.
Custom finish work always costs more than it's worth. Chroming is always double what you expect for a good outcome. JB Weld can NEVER be under utilized and always performs unexpectedly well. GOOP was God's gift to Man, I suspect that he slipped a case or two to Noah . . . can't be sure until one of those folks on the crack-pot religious channels finally locates the Arc. I like to think that they'll find it buried along the Harlem River having sailed all the way to the New Jerusalem, where the flood waters receded depositing Noah and his boys within a short journey to Borough Park.