From the angle of your pic, I couldn't tell if a seal was present. Mine does have one, attached to the tranny & the yoke slides in, it angles inward from the rear face of the tranny tail to the driveshaft, extends out maybe a quarter inch. This was included in my measurement.
There is a little over 4 inches of available spline area on a standard T5 yoke so you have about 2-1/2 inches of splines in contact. With your engine description, that should be enough to be reliable. On my custom drive shaft that I had made, I have 5/8 inch showing to get the most amount of yoke spline contact and the most in the bearing in the tailshaft housing. The axle moving up and down only needs that much to allow for movement in driveshaft and yoke. Don't forget if you have the axle at full droop it will pull the yoke out of the tailshaft housing quite a ways.
There was a couple of threads on this forum a little while back discusssing this issue. It seems that FFR has shipped some drivelines that are short or the engines are set more forward or something. A search for "yoke" will get you some answers. There are several styles of rear tailshaft housing yoke seal. One style has a seal and a rubber "scraper" on the outside of the seal that cleans dust and dirt off the yoke as it moves to prevent junk form getting into the seal proper. That scraper sticks out about 1/4 or 5/16 inch behind the seal. Timken number 5202 rear seal has that scraper on it to prevent dirt from getting into the seal.
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