DieselDog is very correct. You will need a 240 engine from another W115 chassis. All that modification takes it from being a fairly easy swap to one of those never-ending projects.
If trunk, floor pans etc... aren't rusty, that is a heck of a deal for a very clean W115.
How is the interior?

__________________
Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John