The shims are to centralise the followers on the cam as well as reduce end-float, any adjustment needed depends on each crankcase/timing cover combination (assuming the gasket thickness is a constant
Usually (from what I've seen) the shims go behind the followers, yours could have the clearance being taken up by two shims the same size. It's probably not as critical with the single-lobe cam and in any case it's also relatively easy to move the cam bushes slightly, which can effect where the cam runs (there are also shims available for the camshaft, to reduce end float, but many people just close up the bushes slightly to get the same result).
(I've seen more engines without any shims than with them so I'm not sure that it's necessary to lose a lot of sleep over it unless there's a big disparity in the positioning of the components - but maybe trying to line it all up would be a pleasant diversion from the usual Xmas mayhem
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)