What is wrong with having an 86 305 a two bolt main or should you swap in 350 w4 main?

The problem with a 2-bolt main 305 (or any small-block Chevy with 2-bolt mains) is that it's inherently weaker than a 4-bolt main version. The two-bolt design provides less clamping force on the main bearing caps, making it more susceptible to main bearing failure, especially under high stress (like high RPM, high horsepower, or nitrous oxide). This weakness limits its potential for modifications and high-performance applications.

Swapping in a 4-bolt main block (from a 350 or other applicable SBC) is a common upgrade for this reason. The 4-bolt main block offers significantly increased rigidity and strength, allowing for higher power outputs and more aggressive driving without risking catastrophic engine failure.

So, it's not that there's something inherently *wrong* with a 2-bolt main 305 in a stock, low-power application. However, its limitations severely restrict its modification potential and reliability if you plan on increasing power or stress. The 4-bolt main swap addresses this limitation directly.