I have a 90' formula with a tbi 305 and 700r4. I'm thinking of doing some engine mods: headers, cam, pipe, maybe heads too. Unless I find a cheap 350.
I just want to know how much power I can have before I start breaking things, and should I buy a new rear end and tranny before I start fixing up the engine?

When they break.

Or, whenever you're afraid they might.

first off hom many miles do you have
on the car and how do you drive do you floor it every chance you get like me.THats why im going with a t56 its more than enough for a stock 305 tbi but im also going with a 350 LT1 later in the years probally 2 yers later when i get more money.

You should be okay until you get some sticky tires. My rearend lived much longer than I thougt it would but eventually I ripped a tooth off of the pinon gear. Put in a Currie built ford 9" now I do not worry about it.

MD

Even if you Mod the 305, I do not think you will have a problem with the tranny or rear axle. That is unless you do something Dumb or Abuse the car. Same thing if it was a 350 engine.

I disagree slightly with this. If your modding the 305 i doubt youll have a problem. From what i understand your tranny and 10 bolt wont have any problems until you get to the 300-350 hp area which i doubt youll do with a tbi 305. However everything has a life span and you will definitly be shortening the life of both.

However if you get a 350 and start doing the same mods you were just talking about youll definitly have to start thinking about swapping out your drive train.

Personally if i were you what i would do is figure out where you want to end up hp and torque wise when youre done modding your car. Then since you know how much power your car has to be able to handle go out and buy the according pieces ie transmission, rear end. Then when you know your car can handle your future power requirements you can save up and drop in the motor you want all at once and you wont have to worry about breaking anything.

If you are hell bent on doing engine mods right now id start with things that are transferable to a bigger engine ie exhaust, intake etc.

I will say im on my 4th 10 bolt in my 84. It is a stock L69/T5 combo. I finally got a set of Richmond 4.10's and now Im not too worried about it...well, maybe a little. 10-bolts SUCK!!!!