The after WWII model, namely 640, used three different actions. I listed it all in a previous post, but fast, fast here what it is; the early 640 were based on a "commercial" M94 (bent handle, but "solid left side wall), just after, when they were available again, they used the FN made ""C-Type" M98, then, later the FN designed M98 "H-Type" action. All of them were available in 9.3X62, 8X57JS, 6.5X55, later made in some popular american calibers (only with the "H-type" M98).
The M94/M96/M38 (and the M92, M93 and M95) are basically the same action. Before the M98 all Mausers were ####-on-closing. The real difference between the three swedish-made Mausers were the handle, the M38 was actually a M94 Carbine (with the bent handle) with an intermediate barrel lenght (610mm vs 450mm for the M94 and 740mm for the M96 (straight handle)). There is also a variant, called M96/38 with straight handle and rebarreled to 410mm. Husqvarna made the M38 and M96/38 and some M96, too.
There was some special run made by Carl Gustav for target use (before the coming of the CG63) wich used the solid wall receiver, too (but they are very scarce).
As for the strenght of the M94/96/38, most experts agree that the small ring actions, before the M98 (some also say before the "upgrade" of the 8X57 from J to JS (and the 10 000 PSI pressure rise), in 1905, for wich Paul Mauser created the M98 in the Large Ring version) where not designed to handle high pressure rounds (wich is logical, the powders to provide higher pressures were not invented yet).
Good reading about it can be found in the Mauser Monhtly Magazine, read the Larry Ellis (Master Mauser Collector and Researcher) answer to the Rifle Magazine Editor's assumptions
http://www.frombearcreek.com/nonfiction/m_monthly/vol_2/ED8VOL2.pdf for the so-called expert opinion and then
http://www.frombearcreek.com/nonfiction/m_monthly/vol_2/ED9VOL2.pdf
for Ellis' answer point by point to the arguments of the Rifle Magazine Editor's.
While the M94/96/38 is not as strong as the M98 (especially in it's Large Ring form, with the addition of the post WWII commercial Brno M98 small rings), in it's solid wall form it can handle rounds up to 55 000 PSI range.
The 1640 is often referred as a cross-over between the M94 and the M98, because HVA did not have the rights to manufacture the M98 in it's pure form. So they decided to keep the small ring configuration and added a third locking lug to the bolt. This action is, anyways made of the best material (for the time) and can handle modern (post WWII) calibers, having a MAP up to 63 000 PSI.
The 1900 is a completely redisigned action, very close in design to the Tikka action.
Hope this helps.