Bigblock in the twisties
 

Im thinking of taking on a project, but Id like some info on what the outcome is likely to be before I throw money away. Looking into stuffing a 429 that is still yet to be built into a foxbody mustang, likely mid to late eighties. Prolly keep it an auto too. How bad is this thing going to handle? I like straight line acceleration but I still want to be able to take a corner without severe understeer or oversteer.

You're outta luck. :D

With a bigblock, you'll have extra weight to deal with, on an already noseheavy car. If you modify the suspension for dragracing (light springs) you'll have a real tough time driving any corner fast. If you do stiff springs to support the car, you'll do ok, but no canyon-carver...

Decide on what you wanna build, then make a plan, and design the whole car to that plan. Dont peicemeal parts of the car for multi-designs, you'll end up unhappy.

Drudis is right. Unfortunately with cars you usually cant have the best of both worlds. Its one or the other. If you have a car set up to drag you wont be able to handle as good as someone who has a road race car. And unless you can lighten up the front end and the 429 up alot your going to be plowing into corners.

You would have to totally re-design the chassis and suspension in order for it to handle the way your're looking for.
The chassis would have to be designed for the engine to set back further to compensate for the ectra weight. Then your talking a beefy suspension to handle the added weight and traction problems you would have with all that torque.
I am not an engineer but that would be my guess as a start.