So, What about if I got a coupler that mated the keyed shaft on one side and a six inch section of the output shaft from the original transmission? This would be able to slip into the original driveshaft. Would it be an issue strength wise being this length?
Your problem with that is ensuring that it was well enough mounted to the motor driveshaft - it would need to be very rigid to avoid it whipping
I would prefer the driveshaft with the "plunging" joint in it -
I agree with Duncan. That's why I listed this alternative as "probably not structurally sound" earlier.
Also, would it be an issue of the output shaft being in the slip yoke without the collar that was on the transmission tail piece?
The usual tailshaft housing surrounds the sliding bit of female splined shaft, as a protective cover; the equivalent sliding section in a shaft typically has a rubber boot. Yes, protecting the sliding section from getting dirt in it is an issue.
So this is a full drive shaft or a segment piece to complete the length from my motor to the existing drive shaft.
... you may be able to just buy one the right length - or move your motor back or forwards until the length works out
Many long trucks have multi-part propeller shafts, with a joint and steady bearing and support bracket at each connection between shafts. This shouldn't be necessary in a car, especially if you can put the motor back in the transmission tunnel, but many current cars do use two-part prop shafts.
Using an "off the shelf" one has a lot of advantages
Most older cars had propshafts, today BMW and Subaru and various trucks have propshafts
Find what there is - if you are lucky you can buy a new one or get one from a scrapyard
Don't forget the space needed for the adapter that goes on the drive shaft
Anything with an engine and transmission in the front, and driven rear wheels, will have a propeller shaft (so yes, almost all BMWs and pickup trucks are examples); however, many won't have a sliding part of the shaft, either because they use a sliding spline at the transmission output shaft (like the '54 Ford), or because they use constant velocity joints (instead of U-joints) which allow plunge, just like the axle shafts used with independent suspensions.
A live beam axle (such as in this 1954 Ford) will typically require more shaft length change than an independent rear suspension (in which the final drive unit basically stays stationary). With IRS, the shaft might even get away with the slight length change allowed by guibo couplings, but you wouldn't want to depend on that with a beam axle.
In a quick search, it does appear that BMW tends to use shafts with a sliding section. Here in Canada it's hard to imagine a more expensive source of parts than German luxury-performance auto manufacturers, but of course this varies by location.
All-wheel-drive vehicles will have a propeller shaft (except for electric vehicles, and hybrid vehicles with electric-only drive to the axle at the other end from the engine); however, that shaft may not be designed to handle a lot of power.