Shorter rods would bring stock pistons lower in the bore and may cause conflict with the crank counter throws at bottom dead center. Using shorter rods would work if the piston pin position was changed for the longer crank throw. A longer rod with stock pistons would conflict with the deck [head] at top dead center. It's just that the overall length [piston and rod combination] would have to be such that it met the demands of the cylinder and crank dimensions. You'll see some really short pistons in stoker racing engines for this reason.
'95 R.R. Classic LWB
'76 Series III Hybrid 109
'70 Rover 3500S