I have been an inline six fan since I got my first car, and truck around 16 years old both with sixes in them. I haven't ran a six for years and now have a daily driver with an old flat head six and a 235 with Corvette side drafts waiting for my truck (next build).
To put the little later and much better 235 in your car is a bolt in. The down side is if the rest of your drive train is in good shape and doesn't need to be replaced or rebuilt. I took a gamble with my daily driver and I am having a ball driving it every day. These old cars can last forever if maintained anywhere right. So your trans and rear end may very well be perfectly good and you bolt in a later 235 and have at it. There are tons of speed equipment and such so you can get a few oats out of the little 235. You put this later motor in and do the brakes and change the lube in the rear end and tranny and drive the wheels off the thing just like that.
Or you may find yourself wanting to change out the rear end and tranny.
I had a basically stock 1959 235 with a mild cam and a home made intake with a four barrel and headers in my truck. It had a Muncie four speed and a 3.90 55 Chevy rear end and it ran 16.40 in the quarter at 85 mph which is VERY respectable. It doesn't sound like much but if you go to a real drag strip with some normal street driven muscle cars and stuff like that you will find they don't run as fast as the numbers you see people bragging about!
So, here is the scoop, changing out your rear end and tranny may be needed just to get it up to par. If that's the case, still super easy stuff. So if you changed out that old 216 and put in a later model 235 or the 62 and up 9 main bearing 250 or a 292 with a T-5 five speed (cheap as hell) and a little ten bolt rear (cheap as hell) you can put together a very cool car with plenty of poop for pretty cheap. It will all go in very easy, easier than a V8 swap and you have something interesting, you have a "Stovebolt" Chevy and it will be fun to drive and fun to open the hood at an event.
These late model sixes are basically a "Small block Chevy" in simplicity and parts interchange. There are PILES of performance parts for them. And to rebuild it is reasonable.
Check out Inliners International devoted to all Inline powered vehicles
I just ran into a bunch of these guys at the last local show, there are active groups all over from this club.
Lot's of guys Stovebolt.com Stovebolt -- Old GM - GMC - Chevy Trucks!
into the sixes as well.
I have to tell you, I LOVE driving my little flat head six (1959 Rambler) every day. This particular car is GUTLESS I mean REALLY gutless with a little 196 flathead but I don't care, I love driving it because it's different. Not only different from all the other late model cars on the road but different from the other vintage cars I run into too.