We had to fabricate a new jackshaft when we tried 4 x Webers. Wasn't that involved. Used x 2 bearings only as worked out (with chrome moly tube) that whip unlikely below 7000 rpm, short shaft and tubular. Brackets attached either end. If you know the thickness and material there are plenty of critical speed calculators on web to work out where / if it stock jackshaft likely to whip although they thought it would under full load / off load situation. Sounds like Johnny has hit on why centre bearing often fails. We had thought of doing what BW has suggested and having a nylon bush that would control the whip but give clearance at centre. Never got round to it as we replaced the Webers so went back to stock jackshaft.
Johnny has some good photos on his "harleywax" website dealing with jackshaft refurb.
cheers
Larry