LINK:
TractorData.com Ford 950 tractor information
If you are buying new:
Disc Harrow widths are measured at widest part with the gangs "straight". They are never used in the field this way as they would not cut. To cut, the gang angles are swept back, the front more than the rear, decreasing the effective width. So, you need a Disc Harrow which a manufacturer would promote as 8'-0" to 8'-6". It will be a bear Discing with that tri-cylce tractor with manual steering.
Plowing and a SOIL TEST and soil amendment with deficient micro-nutrients well worth the time.
You will need a Disc Harrow with 20" diameter pans, cutting 6" to 8" deep to reasonably flatten the furrows from a 14" plow. I do not think the Ford 950 can pull a Disc Harrow with 22" diameter pans on a second pass, when the soil is fluffy and the tractor and Disc Harrow sink in.
A Disc Harrow with rear pan spacing of 7" or 7-1/2" will smooth after the plow better than a disc with 9" rear pan spacing.
If you buy used, make sure the Disc Harrow is not so banged up, not so rusted, so that it is impossible to adjust the front and rear gang angles.
You will need to disc twice, at 45 degree angles, then drag repeatedly to get the field smooth. I would take time to get it smooth so the field does not pound you indefinitely into the future. (Tractor owners inquiring how to smooth fields/pastures is one of the most common threads here on T-B-N.)
Contemporary seeds are much more productive than the old stuff ~~ but you have to fertilize to obtain the greater yield.
Consider if aerating/ripping the pasture, then amending and overseeing, might meet your needs and be less work. (I am old. Thinking of manual steering and multiple passes makes my shoulders ache.)
LINK:
http://www.tractorbynet.com/forums/attachments/308251-disc-harrow-selection-18-45-a.html?highlight=
I hope your 45 year old tractor does not become a money pit.
Photos: Dirt Dog (brand) APP/Field Cultivator aerating Bahia pasture, growing on Florida sandy-loam.
VIDEO:
Cultivator/Ripper - YouTube