Congrats on your new acquisition. A 2nd gen FB to play with. Lots of parts availability for ya, as long as you have the "long green". (Or in todays currency, would that be bitcoins?)
Anyway, you went from one end of the tranny spectrum, to the other. A 465 is practically indestructible, and a T5 is made of glass. Not rated for anything more powerful than a snail on crutches. But as long as you keep it a 2bbl carb., and don't push on the go pedal too hard, you'll; probably be alright. At least that's what I've been told. I have 2 T5s. A WC, and a non WC. Haven't been able to test them out...yet.
You mention something about the rear mount, and made me think of what I had-ta do, to make mine work.
If your T5 is from an 80s F body, then for some God forsaken reason, GM engineers decided to tilt the tranny CCW about 7° (?), and the bolt pattern on the chebby bellhousing provided that angle. But they tilted the tailhousing mount back the other way, to make it fit the flat cross member. I'm using a WC T5 behind my light duty 301T motor, (in an 84 T/A) and used a conventional BOP bell housing, which necessitated a "wedge" to bolt it to the cross member. Sounds like yours might be the same way. It may sound bush, but it's the logical option, compared to others. At least you got a stick shift.
Here's a photo of my rear mount
It's not pretty right now, but in the "experimental" stages. You'll notice the problematic torque arm that also needs ta be hooked to the tail. Fortunately you don't have-ta deal with that.
Anyway, good luck with your project. As for driving it W/O 5th? If it's not making any noises, you should be OK. And with a 2.41 gear, (?), who needs an OD anyway?