New to Mopars---BUT always a cool different car fan.

short story - ended up with a 66 Barracuda to restore for grandson as his 1st car. I am at the end of the build and have some issue with the car/transmission/rear end power...
former owner put in a A-833 4 spd with OD behind the slant 6.. (it still has the stock 7.25 rear end with i believe 3.23 stock gears)... pretty sure he never had it running with this configuration.

This car is sluggish in 1st and 2nd, and I actually lose speed when shifting into 3rd.. (never have tried to get up to speed for a 4th gear shift)...
Im hearing that this may be too tall of gears for the rear end, and off of another thread it might be the linkage hook up incorrectly for 3/4 gear. (tab goes down?? not Up.)

anyone on the site have some additional thoughts on what to 2x check? (I'm almost at wits end.)

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Me and Gavin 3.8.2022.jpg