The search here is your best friend. See the word search up top here? Click it and enter "250 swap" (with the parenthesis), hit submit and you will have many pages to read. You can PM the guys who have done it from there. The internet will be, likewise, full of info.
Example of what's in Stovebolt:
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=948368
And one with a mention of Langdons:
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=575613

The 250's are good motors. Sounds like you already have open drive, that's good. Do you have 12V yet? GMC was late on that and has some positive ground years. None of which is a problem to fix. 55.2 for Chevy 12V and maybe 56 for GMC?


Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn