I have an 86 iroc with a tpi 305 ci 4 spd auto trans with od and rest of car is stock. I want to swap out the 305 for an ls3 sbc. while the engine is pretty straight forward, I have no idea what else I would need. do I need new motor mounts? if I do, are there after market mounts that would fit this swap?

 

Try here. Lots of info in the stickies.
https://www.thirdgen.org/forums/ltx-lsx/

 

Welcome aboard thirdgen.org.

The stickies cover the basics. Yes, you need aftermarket engine mounts or adapters. And custom exhaust.

Do you have a source for the LS3, or just know you want it? The factory has complete engine/transmission/harness/control system kits available for about $12k. You'll still need the adapter mounts (engine and transmission) and exhaust.

Holley (Hooker) has come out with a pretty cool kit you can put together for mounts, headers, and exhaust. https://www.holley.com/blog/post/hol...n_gm_f_bodies/ Even though I've already done the swap, I'm considering upgrades (including the 525 HP LS3 crate engine) and using Hooker's stuff.

More details on your plans, goals, budget, usage, skill level, etc., would be helpful.