Yes, you could rebuild your block yourself, but that would mean a lot of down time, which is a problem if you use your vehicle all the time.
If you buy a short or long block already rebuilt, you can do the entire swap in a weekend. I did it before. I bought a stock rebuilt 258 block, bought all my performance parts (cam, intake/exhaust, valve cover, holley carb, etc. from Clifford Performance), and put all my performance parts on the block in the garage at my leisure while driving my tired motor. Also a good time to buy all new water pump, fuel pump, etc, obviously! Then, when I was ready, I started on Friday night, pulled my old motor, and started swapping the tin/covers, etc. that I needed off the old block. Then, by late Saturday, I was ready to drop the new motor in. I had it in and running nicely by Sunday afternoon.
By Monday, I was back in business, which was important, as it was my daily driver.
Of course, I had to drive it with "temporary" Cherry Bombs bolted to the Dual Exhaust Header Collectors for a week (for backpressure, so you don't toast your valves), LOL, because my appt. for my Custom Dual Exhaust System was at the end of the week for Friday night/Saturday.
Anythings possible when you put your mind to it, huh? LOL