When i also searched for short plugs before two months,i talked with a NGK dealer in the states and told me that the R5300 series are discontinued from NGK.
R6120 is the old code of R7282 so they are equal.
He told me that if i want short plugs i must buy the R7282 series as these are the only short plugs that NGK produce now,and the only way to find R5300 is from an old stock.

Thats what i remember from my conversesion with a NGK dealer in Ohio.

You maybe change your plans and buy R7282 which are more expensive,except if you know where to find old stock R5300.
R7376 are not shorty.

regards
Stefanos