The 350 will of course drop right in and everything from the 5.0 liter will bolt right up as long as you stay in the same generation of engines. The down side is it usually requires a gear change in the outdrive to allow for more H.P. You *MAY* be able to get by with a change in props depending on the boat but just be aware you may not.
With a frozen / cracked block the manifolds, intake and exhaust, and the risers are immediately suspect and you need to test them for leaks once off the engine. The water circulation pump is also suspect but to be honest there is no way I would put an old water pump on a new engine, they aren't that expensive so replace it. If that's too much for the budget at least put new bearings in it with a rebuild kit.
While you have the engine out you might want to take advantage and install an oil drain kit on the engine. You'll be thankful every time you need to change the oil. It's also a great time to take a hard look at the gimbal bearing and replace if needed. Of course cleaning up the bilge is a given but you might consider a can of Bilge White. Amazing what a little paint can do! It's also a good time to sand smooth and repaint all the drive pulleys (alternator, power steering, harmonic balancer, etc.) A couple of coats of quality paint and your good to go.
Be aware you MUST have and USE a drive alignment bar. You can find them on eBay or have a local machine shop make one for you. Even a slight misalignment will cause excessive wear of the engine coupler, gimbal bearing and u-joints in a very short time and you will be pulling the engine again.
As to the outdrive, I wouldn't replace it unless I had too. Check the oil in it, do a pressure / vacuum test and if all is as it should be put it back on and run it. It's not hard or time consuming to replace it at some point in the future if needed. These drives are built to last a long time and most of them outlast the boat and engine.
