I'm sorry if I have confused you. Your ideas for your son's car is covering a lot of territory.
The 230-250 s a totally different engine from the 216 and even the 235 & 261. Nothing from your stock '40 running gear is easily useable. I'm not saying it can't be done but it may not be worth the effort. Here is an add for 230-250 mounts and be reading if you can see some of the issues. LINK

Here is an article about installing a 235. Patricks in no linger in business but the information in good.
LINK

My suggestion is to look up similar links on the net and read as much as you can. Find a link that covers what you decide to do and follow it. Personally I would get the 216 running. That alone would be a good experience for your son. If he wants a more modern engine that can work with the stock running gear I'd chose a 235. There are other rear end gear choices that would make highway speeds easier.

"I wonder if God created man because he was disappointed in the monkey?" Mark Twain