It has been my experience that aspect ratio on the tires/wheels makes a bigger difference with the "harsh" feel than the actual springs. If you are doing aa frame-off on an E-Bird, I imagine you are planning on keeping a fairly stock appearance, with 70 or 75 series 14 or 15" tires.

Given that assumption, I would not expect the heavier springs you note above to be too bad, although certainly a noticable difference. With the higher spring rate, though, you will likely need to remove a partial or full coil (given that the static heights are the same) to maintain the "stock" ride height.

I would go for it. I was shocked how well my '56 handled with the rag (bias ply) tires that it currently has. I can certainly understand why people liked to race these cars...

Done rambling now...others may have a different view, this has been my experience.

BIGREDTODD

I replaced the front springs on my 55 Tbird heavier springs. I do not recall the lbs/inch rating but I believe they are 450 lbs/inch. I did not cut them and with the 215/70 15 inch radials the front fender wheel arch is just under 27 inch off the ground. The springs are made of .618 inch wire. I have front and rear sway bars and KYB gas shocks. The ride is not harsh or choppy.

The 57 Ford shop manual list the 57 Tbird springs as free height of 14.99inch.

I put 620 lb front springs in a 66 Shelby GT350 that had factory springs ( 480 lbs?) and that was an acceptable ride also. If your cars front end was as worn as mine was before the rebuild, that alone will make a whole world of difference. Pete

Cheers... Brian

PS: Is a 7/8" sway bar sufficient or would 1-1/8" be a better choice?

Cheers... Brian

To handle well in a corner - the A-arms will have to assume a position that holds the outer tire upright (squares the tread to the pavement). So the outer spring deflection (upon weight transfer) has to result in a movement of the arms that complements the body roll and leaves the tire in the proper position. Ideally - the lower arm should be dead-on horizontal so the hinge point and knuckle (ball joint) on the upright are in a straight line at normal ride height and wheels aimed dead ahead. As the body rolls, the upper A-arm should rotate to move the upper end of the upright to just the right angle to complement the body roll. Getting this just right ain't too easy with a 55's set of suspension hangers.   

If the set-up had no springs (sollid bars) and the tires were locked upright - the car could exert considerable handling side force - but the ride would be terrible. Height of the C.G. would then limit the speed (gross side load) the car could tolerate before it slipped sideways (or flopped over). This is one of the reasons it always "feels better" right away when guys lower their cars with cut coils - springs get stiffer / C.G. goes down.  

Lowering the C.G. with no change of stock A-arm position will give you better handling without increasing spring rate or changing the ride. The most straight-forward way - is a lower aspect ratio tire - C.G. goes right down, tread gets wider (more grip), ride remains the same. There is a limit to this - the tire sidewall is a "spring" - and going to a rubber band on 22 inch rims doesn't get it done...... Second choice - use an upright with a revised spindle position - the Granada or Versailles disc brake mods get a 1 inch+ C.G. drop with no spring or tire change. Add a low aspect tire to this and the C.G. will be right down there....with the original T-bird ride.