I am looking at building a longtail, i think i am going to use a love joint or something similar for the connection of the shaft to the motor. WIll this conncetion work and what size shaft do i need for a 7hp motor, about 55" in length? Very similar to the Stump Jumper Motor

Or where can i find the shaft that will fit a 6 3/4" x 5" prop?

You can go to MCmaster Carr and order a 60in 3/4 shaft of cold rolled steel and thread it yourself. Thats what I do.

Thread size: ¾”-10 NC, should i just ask the machinist for this size thread which should fit all mud props? Also, what is located behind the prop inbetween it and the shaft housing, or do they touch?

what is a love joint??

Love joy coupler...my mistake. It's just a keyed coupler for shafts

use a nut in front of and behiend prop, also put a second nut-nylok- behiend prop.

I'm not sure that a love joy coupling is the way to go, first of all it must be in perfect alignment for a love joy to last, if not you will be changing the rubber insert every few weeks. They do last well if aligned with a dial indicator to within 2 to 3 thousands. If you use a love joy you will be running your motor at about 10-15 degree angle sloping backwards. If you are going to use the 55 inch shaft then it will be more on the order of 20-25 degree angle shift sloping backwards.