225 or 318

There's quite a bit of stuff to change, and it depends on the year of the car and the year of the engine, which you didn't post.

You need to be careful WHAT SORT of 318 you got, as there are at least 3 separate engines.

The old "polyshpere 318, so called "Poly" and incorrectly but widely (pun intended) called "wide block," which were never actually used in A bodies

The LA 318, modeled after the 273, with parts interchange between that and the 340/ 360 to some degree

AND the later magnum engines, which used different heads, intake, and oiling changes, and roller lifters, and while they can be swapped in similar to an LA, it's important to research the parts interchange.

If you have a stick (3/4 speed) you'll need a different bell, flywheel, and clutch. If you have an auto, you'll need a transmission which fits the small block bell. This can be either a 904 or the larger 727, but the 727 will require a different driveshaft, or shorten yours

Depending on the year of your car body, the transmission can be an issue. Is yours old enough to be a push button? Rear fllange type driveshaft? So what year is it?

And be careful of the transmission you buy (torque converter) as the early autos for the 273's had a smaller converter register, or if you find a later 360 trans, you'll have to knock the balance weights off the converter, or replace same. Also watch for "lockup" transmissions

You'll either need a V8 crossmember or special aftermarket mounts made by places like Schumaker to fit the V8 to the /6 crossmember.

Depending, early V8's had the bottom radiator fitting on the opposite side of 69? later cars, and the newer pump is a little longer. Your radiator may or may not be large enough.

Of course you'll need proper exhaust manifolds, many off the other car/ truck bodies won't work, or you'll need headers.

S' all I can think of for now.