Yes it’s been done. Search on here & you will find it. The slipper clutch pack is deeper than the 250’s, so if you use the 250 cover you need to remove the case reinforcements on the inside to make clearance for the clutch depth. A simpler solution is to use a 300 clutch side case as well. I believe they are the same where they meet the engine, simple check method is to buy a 250 & 300 gasket to compare. There is a thread on here that shows pics & differences between the new gen 250 engine & the 300, in full strip down.
I put a new gen clutch in my pregen engine, works great & does away with the death beat of the pregen engine
Have fun.