Can a1975 Yamaha DT250 rear shocks be rebuilt?

SOURCE: rear shock sealhead Kyoba on Y Z 426 F

I assume you have disassembled the shock, ground off the peened end of the shock shaft, removed the shaft nut, and have the seal head off already? If so I recommend not to disassemble the seal head itself. The fragile little bushings and seals in the inner diameter of the seal head are extremely difficult to R&R without scarring the seal head or the replacement parts. Your local dealer can order a complete seal head from KYB Direct or Moose at a reasonable price. This is a much easier and more reliable fix.

SOURCE: whats the pressure settings for the rear shock.

the pressure ranges from 0 lbs unloaded to 57 lbs heavily loaded.
The info is in the manual

SOURCE: Ineed to replace rear shock on rsv 2001.

Both Ohlins and Works performance make shock/spring replacements for the Venture, an internet search should get you a number of sources.