You could switch out the billboards for other items that have a bigger mutliplier, so that you don't raise your item numbers but still increase your multiplier. Of course you'd ultimately lose the multiplier from the stored billboards.
Personally I think Lard Lads are quite bad for farming, because they take up so much space. Maybe you should at first think about how many millions of ingame cash you really need every day and build up farms of blue/white/pink houses to earn this money. That should in the end save you some space. Or you look out for the rare multiplier items which earn you ingame cash, like Wailing Walls, Elf Houses or Snake-Trees (forgot the actual name). Normally these are quite expenisve and only available during a short period of time, but with your set up and multiplier it should be no problem to build up a (small) farm.
