At the top of this pager there is a text-link to "TECH TIPS". Click on it, scroll down the page to the Engine section. There are two good articles on swapping the old 216 to a newer 235.

It is a very easy swap to do.

Off the top of my head there are only two things that you've got to deal with-

The waterpump/fan etc... the 216 was different from the 235 in terms of mounting location and how far it protruds towards the radiator.

Also, you MAY need the front engine mount from the 216 to use on your 235. Some 235s had side engine mounts and didn't use the front engine mount, so you'll want the front mount from your 216.

Save the vavle cover from your 216 and the two head bolts that hold it in place. You could put those on your 235 and make it look like a 216.

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi