Welcome to the site Krista.
Without the door tag we can't find much information about your car but this is what I have found with the number you gave us is:
Year: 0 1970
Plant: F Dearborn, MI
Body Series: 05 SportsRoof, Mach 1
Engine: H 351 2v V8
Unit: 134285 134285
The engine number 9H598143 shows that it's a 351 (H) manufactured in 1969 (9) so it could be the original one. You can purchase a Vin decoder disk. I never tried it but it seems very usefull.
http://www.mustangdecoder.com/disk.htmlGood luck and if you want to show your Mustang in its actual condition, please feel free to do so.