I have just swapped out my 7 inch rear end fron a 65 and installed an 8 inch 5 bolt to accommodate a I6 to v8. I am using 245 45 17s. Question is will the old 4 leaf springs be strong enough. They were newish at the time of the swap.

 

They should be fine. If you didnt have issues with them before, there should be no reason they would cause an issue now. As long as they arent sagging and you have good shocks, you should be good to go.

 

I'd have to agree with boogerschnot (did I just say that?).
The springs have more to do with supporting the weight of the body than the axle.

 

Thanks guys. I had a bit if a moment when I realized the top clearance under the well was also reduced but I have good performance shocks. So fingers x-ed

 

Just an observation, but if I were running that size tire and wheel, I would install a set of 4 1/2 or 5 leaf standard eye springs (from Mustangs Plus) just to prevent tearing up a quarter panel or wheel the first time I decided to throw down a monster launch. In the grand scheme of things, rear leaf springs are reallly pretty cheap. Ride will be a bit rude, but you can't have everything.
Best,
Al