I have some conflicting load data.

My Speer manual lists the maximum charge with 55gr FMJ-BT bullets at 28.0 grains of 748, which seems a bit on the heavy side.  My Hornady data lists the maximum charge at 26.4gr.  The Hornady data has the bullet being seated shorter (2.200") than the Speer data  (2.215").  I know that components can vary some manufacturer to manufacturer, but my main question is this: are Speer's 55gr FMJBT bullets the same dimensions, length-wise, as Hornady's?  In other words, if I'm using Hornady bullets is it safe to work up my load based on the Speer data, or is there an important difference between then two that makes that unadvisable.

Start with the lower of the 2. Even 26.4 sounds hot to me. I've had very flat primers shooting 26.0 in hot weather with 55 V-max bullets. I've since reduced my load to 25grains with 748 and 55's.

Sorry, but I don't have the 2 bullets you mentioned to measure. It doesn't matter though. Start low and work up. It's all about what works in YOUR rifle.

I've used a lot of 748 with Win, Hornady, military surplus, RP and  a couple other  55 fmj bullets. For a good load that will reliably cycle your AR, is easy on brass, shows no pressure signs for me in my rifles, try 25.9 -26.1 grains of Win 748 powder. I've had no problems with high outdoor temperatures loading at this range. I wouldn't go above the 26.4 grain level. As far as overall length it depends on the bullet you're using. I'd try the length recommended by the bullet manufacturer. Some 55 fmj's such as Win and military bullets have a long sharply pointed tip and overall length would be about 2.250" while some have a blunter rounded tip and overall length would be from 2.200" to 2.230". My opinion of the Speer data at 28 grains is that it is dangerous and should never be used when substituting another manufacturers bullet and I've not loaded Speer 55 fmj's and don't know if it is ok with them. While Win 748 is one of the good .223/5.56 powders I've found I prefer H335 for 55 grain bullets.

I've found the new Hornady .223 data to be low - at least in my rifles, which is reflected in much lower velocities than Hornady shows, but I haven't tried W-748.  Perhaps a newer lot of 748 has a slightly different burning rate?  Here are some examples of previous Hornady data:

Hornady #3 - 55-gr - Max 27.7 grains

Hornady #4 - 55-gr - Max 27.7 grains

In addition Hornady uses odd powder charges to "fit" an even velocity (i.e. 3200 fps - 3300 fps).  If the next 100 fps. velocity step powder charge is too high only the 100 fps lower velocity is shown in the book - which can be somewhat lower than a real maximum working load which would fall somewhere in between in velocity, if you get my drift.  

Speer uses even powder charges (i.e. 27.5 - 28.0 etc.) and gives an odd number velocity (i.e. 3143 fps etc.) which is shot and recorded from an actual rifle, not a long pressure barrel as an additional check for real world performance and pressure.  If the next 1/2 grain step is too high, then Speer uses the lower "even" step.  
This alone can account for the 0.3 difference between Speer data and the older Hornady books maximums.  It is just a matter of tradition and philosophy between the different sources on how they do things.

I would work up and use the Speer data as a ceiling watching for pressure signs.  If you can chronograph and know what velocity to expect from your barrel length - do not exceed the norms for that barrel.  That is for a 20" AR you probably should keep loads under 3200 fps, or just stop when it functions and groups well.

Wow!  With 27.0 gr of 748 behind a 55 gr fmjbt w/c bullet, I'm getting M193 velocities.  3200 fps out of a 20 inch barrel.

I rolled my load back to 26.5 to accomodate the temp sensitivity nature of 748.

27.5 grs of 748 is a standard milspec  velocity load, it's been used by many people for over 20 years

That sounds about right, but with how temp sensitive 748 is, I'm not comfortable at that level.