Regarding sizing, you'll find that your O.E. tire was an 8.00-14 which is 8.00 inches wide (section width).The sidewall is 83% as tall as it the tire is wide (aspect ratio), and the "hole in the middle" is, of course, 14" for your 14" rim.
Thus, the important measurments here are 8.00" wide, and 27.28" in overall diameter (from the ground to the top of the tire - derived from the two sidewall heights added to the 14" wheel diameter). Because P-meteric sizes have replaced the old numeric sizes, the closest thing you have available is a P215/75R14. This is 8.46" wide and 26.70" in overall diameter. This is really your best bet, and I would strongly recommend you go with this and nothing else.
I run the slightly narrower, slightly shorter P205/75R14's on my T-Bird, but it does lower the car a little too much and create some wheel-well gap. Because your wheels are only 5.5 inches wide, any tire with a section width higher than 215mm (8.46") will be too wide and would potentially have problems on your skinny 5.5" wide 14" rim. A good P215/75R14 such as a Michelin XH4 (if you can still find them - they were discontinued about 2 years ago but quite a
few still remain) would probably be best. I wouldn't go with a P225/70R14 either, as that would be too wide for your rim, and it would be over 1/4 inch lower than the P215/75R14's.
225/75R14's have largely been out of production since the early 1980's, and even if you could find some of those, they are still too wide. Of course, 215/70R14's, 205/70R14's, and 195/75R14's are WAY too small.
Another important thing to remember is that you'll be going from bia-ply tires to radials. Going with radials is a very good thing as bias ply tires didn't ride, handle or last worth a crap compared to most modern steel belted radials.
However, remember that your car's suspension is set up for a bias ply tire (stiff sidewalls, not much flex, etc.) and that since Ford designed your T-Bird to use it's ultra-soft springs to make the ride softer, it relied on the ultra-stiff, non-flexing sidewalls of the bias-ply tires to re-gain some of the cornering and handling ability that was lost with the cushy springs.
This is opposite of modern "radial-tuned" suspensions that nearly all new cars have, as they rely on stiffer springs and shocks for better handling/cornering, and the softer, more flexible radial sidewalls of the tires to act as a "front-line shock absorber" for smoothing out the ride.
THIS MEANS that your car will, unfortunately, handle a little differently on radials, as they will have more sidewall flex (unless you go with a very high speed rating which won't likely be available on any sizes you'll need anyway). Just make sure you run at least 30 psi of air pressure in the radials (but don't go over the max, either - usually 35). The O.E. pressure spec (26 psi) was for a bias ply tire with sidewalls much more stiff, so the radials need a little more pressure to get back some of the stiffness.
All in all, however, the radials won't ultimately handle much worse, as the tread compounds are FAR superior now than they were 38 years ago, AND because radial construction by its nature distributes the car's weight much more evenly and puts a larger foot print of the tread on the road.