The trans will hold p just fine. Open versions of the same transmissions were used behind some of Chevrolets most potent engines all the way up to the mid '60s, including Power Pak and Feullie 283s and 327s, plus they were even found behind 396s and 427s as the base transmission.
BTW, I've ran mine, which is the open version, into the triple digits without any problems at all, so 65 shouldn't be a problem.

Bill Burmeister