Here's why:
* Block Differences: The 1991 350 is likely a pre-Vortec small block, while your 1998 305 is likely a Vortec (or at least uses Vortec heads). The block itself may have different bolt patterns for accessories (alternator, power steering, etc.).
* Head Differences: Vortec heads are significantly different than pre-Vortec heads and will *not* bolt onto a pre-Vortec block, and vice versa. The valve angles and bolt patterns are incompatible.
* Intake Manifold: Your 1998 intake manifold will almost certainly not fit a pre-Vortec 350.
* Other Components: The oil pan, distributor, and other components may also be incompatible.
To fix your rod knock: You have several options:
1. Rebuild your 305: This is the most cost-effective option if the block is otherwise in good condition. A rebuild would address the rod knock and potentially upgrade other components.
2. Find another 305: Sourcing a used, good-condition 305 would be a relatively straightforward swap, minimizing the need for modification. Make sure it's a Vortec to maintain compatibility with your existing parts.
3. Find a compatible 350: You could find a later model (post-1996) 350 that's a Vortec engine. This would be more complex but potentially offer more power. However, you'd still need to ensure compatibility with your accessories and other components.
4. Engine swap (most complex): Swapping in a completely different engine is possible but requires significant modification, potentially including wiring, computer systems, and mounts.
In short: Using the 1991 350 with your existing 1998 parts is not feasible. Sticking with a similar generation engine (Vortec) will make the swap much easier and cheaper. Consider a rebuild or replacement 305, or find a later-model Vortec 350 if you want more power, but be prepared for more extensive work.
