I do not have the slightest idea relative to actual dimensional numbers for this specific rifle.
Most of my reference materials are long packed away in an unorganized manner. I am making some educated inferences here from very incomplete info. If you plan on shooting this rifle, I suggest that you test it in a fixture several times, and carefully examine the fired cases, before you put yourself in potential harms way.
I did manage to find James V. Howe's The Modern Gunsmith c 1941. He devotes a couple of pages to the 8mm Lebel in some detail, but no head-space dimensions. In V II p392 he mentions that: "The face of the bolt head is cupped out to a depth equal the thickness of the cartridge rim;"
From the foregoing I infer that short HS is not likely to be an issue. Unless you are going to reload for it being a little long is not likely to be a problem either. Especially, if fixture (old big rig truck tire and a string) firing tests show no evidence of case stretching near the rear.
I would think that the max HS numbers for the .303 British or .30WCF (aka .30-30 Win) minus their respective max ammo rim thicknesses (from a Speer and/or Lee reloading manual) would come out to be within about .002" of each other. If this is the case one should be able to add this average dimension to the observed max thickness of the 8mm Lebel case plus .001" to get a reasonable conservative estimate of the Lebel max HS. The actual measuring of the Lebel HS my be tricky because I think the cartridge head may not be flat. If the rim face of the bolt head comes to within .002" of the breech face of the barrel, I would not expect there to be any problems with case separation or significant stretching.
I just found alleged original French Military dimensions for the 8mm Lebel. in metric.
I CAN NOT AND DO NOT SPEAK TO THEIR ACCURACY! I have little or no experience with, and know very little about any center-fire rimed cartridge rifle.
Old ammo max rim thickness 1.95 mm (0.077") Modern ammo (after 1917) max rim thickness = 2.00 mm (0.079")
Old and New Max HS 2.30mm (0.091") This is a difference (free space) of 0.3mm (0.012") over max rim thickness, and likely as much as 0.014 for a likely minimum thickness rim. I would see how these free space numbers compare with those calculated for .303 British and .30 WCF (.30-30 Win.) which are similar smokeless powder medium pressure cartridges of the late 19th. Century.
Old min. HS 2.00mm (same as new max. rim). Newer (later) min. HS 2.20mm (0.087")
If the above numbers are correct, a newer bolt face recess is 2.20mm deep and there is no more than 0.1mm (0.004") space between its both rim face and the breech face. The old spec bolt has a recess 2.00mm deep and could have up to 0.3mm (0.012") space between the bolt rim face and the breech face of the barrel.
I suspect that if you have a newer spec. bolt and are planing on handloading for it; your case life is going to likely be very short!
Good luck; be careful; and proceed at you own risk.