I have a question- I am dropping a 358 modeled after a zz4 with vortec heads into my 86 IROC- The crankshaft on the stock 305 and my new 358 are different- wil the stock flywheel bolt up and be balanced or will i have to get new flywheel and converter??

Please help- need to figure out fast.

 

Your 86 flexplate should be for a one-piece rear main seal. I'm assuming your new engine has the older two-piece rear main seal, and will require a new flexplate. Your converter should bolt right up to the new flexplate. Just be sure to get the same diameter flexplate (should be 12.75"). You could get a replacement for an 84 with a V8 that would work fine.

 

The ZZ4 is a 4-bolt main, one-piece rear main seal, steel crank, roller cam, aluminum headed crate engine. Your '86 is one-piece rear main seal, which is what dictates the flexplate.

Since you're using Vortec heads, the only other part that could be ZZ4-like is the shortblock. In order to use the ZZ4 cam, it will have to be a roller block. If it is a roller block, it has to be one-piece rear main seal.

My shortblock happens to be ZZ3, but that doesn't change anything about the above (different rods, cam retainer plate, and oil pan drain plug are the only differences between ZZ3 & ZZ4).

I'm with Lo-tec, you don't have the issue you think you have unless the block has a 2-piece rear main seal. In which case, there really isn't anything ZZ4-like about it.