Down right shift pattern. Identification

Hi all.
First post here. But I was looking at my neighbors 74 firebird for sale. He doesn't know much about it because it was a project car from a relative .(I know. Typical story) It has a 350SBC

It has a down lower right reverse 4 speed shift pattern. Some research leads me to believe it could be a truck transmission like sm465 , because the throws do feel kind of long. It also needs a lot of clutch just to get the car moving. Like it has no power down low. I wasn't sure if it was a timing issue or perhaps someone regeared the rear-end drastically juat to work with the 6.55 first gear if it was indeed a sm465

First gear doesn't seem to run out of speed/gear too soon , and 4th seems to cruise around 60mph no problem. It just needs a lot of throttle and clutch to get it moving in 1st. Other than that it seems to drive fine. Except sometimes when I accelerate it feels like a bowling ball in the trunk just rolled and slammed into the left side of the car , with a bang. I'm thinking thats not related to this transmission but Maybe just a bad axle or wheel bearing. Or diff perhaps.

Any one have any ideas of whats in this car before I start bothering him to tear around his car again?

I have attached a picture of the shifter. It is round and straight and comes out of the floor on the correct location.

I am a little worried buying this from him as a project because I don't know how much "custom" work was done to get this odd transmission in there. And if it'd turn into a money pit just to drop a muncie back in.