I read over the mounting for the SB conversion and I shortened a std beetle chassis.
The SB conversion uses the pair of bolts each side to connect to the ends of the arched cross piece (on which the brake master cylinder is connected) so unless the SB conversion kit is produced too short - the short fall isn't there. The chassis shortening (10" or 11.81" depending on if you count the overlap) relies on those same pair of bolts in the front and one either side on rear "shock tower" piece. SEE circled mounting holes to left and right of builders diagram
For years Beck SE has welded the front H beam in place to their tube chassis. I could see you doing that if you wanted the wheels more forward - just remember you want the car lower than a stock VW (most likely) so you need to get castor right (once welded the castor shims, of course, no longer work.
My car isn't near by right now but I know the measurements for wheel base have been listed here before. Just don't recall them.
The wheelbase of the original 356 is 82.7 in (2,100 mm)
http://en.wikipedia.org/wiki/Porsche_356
The original wheelbase of the Beetle is 94.5 in (2,400 mm)
http://en.wikipedia.org/wiki/Volkswagen_Beetle
So with 10" removed you should have a wheel base of 84.5"
