Denis..The more I read your previous posts, the more i hope this machine was a bargain. From the pictures posted the state of it reflects abuse, ignorance and neglect big time by a succession of previous owners.
The crack in the crankcases look to me to be fatigue rather than accident damage, most likely cause being a loose through stud allowing movement and the wear to the casting you now have to rectify. A length of threaded bar was the usual backyard fix, but of course a poor engineering solution.
How you proceed depends on the state of the build. With the cases unassembled, checking for an internal crack is easy, but in any event I would V out the crack and build up a substantial alloy weld web to beef up the lug on both cases. Then accurately drill the through hole to clean it up so that your new sleeve fits firmly, but with no strain on the castings, and a new plain through stud which fits closely to the sleeve ID. What you are trying to do is make the through stud fit the cases with the minimum scope for up/down and forward/backwards movement. This will prevent further damage and also secure the engine unit more positively to the frame, good for reducing vibration.
The difference between inner frame and crankcase is interesting. Is this measured with the rear wheel in place? This may be pulling the rear together, opening up the front. Assuming the frame is undamaged, fitting the rear wheel first means the frame will be under less stress if you simply make larger top hat bushes to fill the gap. The brim of the top hat will clamp the crankcase nicely between the frame members, far better than the usual assortment odd washers most folks use.
I think the reason the bar you showed not fitting across the frame is due to the wear pattern in the holes at the end of the frame. This can be rectified and new bushes made to fit, rather than forcing the frame to where you think it wants to go, and being under strain, a recipe for cracking. If it is as GB suggests, accident damage, you can carefully heat it up and carefully bend it, but only do this if you are super confident and know what to do to control the bend without crushing the frame tube. It is a shame the very dimension you need is missing from the frame drawings. Installing the basic engine unit (crankcase and gearbox) will show where any misalignment lies, use the front mounts and seat pillar/gearbox mount as a starting point, see how the through hole lines up. Aim for a frame and engine in place without any strain.
Swarfy.
Additional. Just done some measuring.
Plunger Crankcase is 80mm wide at lower through stud lug position.
Unassembled plunger rear section of frame,,,,Inner sides of frame tubes are 95mm apart at the frame section join, flat cheeks of the join are 115mm apart.
Not saying this is how it should be, just that this frame section looks to be a good one. So like yours there is some gap, depending on the width of the front frame section joint lugs.
