I ***ume it’s a Chevy? Have you searched for motor mount and transmission crossmember kits? That’s where I’d start.
Buy the ready made motor mounts. They slip over the frame. Read here all the threads that will show you the way. Cast iron exhaust manifolds out there to clear the steering box.
Iron, Give Walton Fabrication (located in NH) a call. He has motor mount and transmission crossmember kits for 49-54 chevys. I used his motor mounts on my 50 coupe.
This kit has been a go to kit for that for a long time. Not cheap but a big headache saver. https://ch***isengineeringinc.com/p...g-kit-for-1949-1954-chevy-non-ce-weld-on-ifs/ Or the bolt on kit https://ch***isengineeringinc.com/p...ine-mounting-kit-for-1949-1954-chevy-bolt-on/ They use the biscuit mounts that many Ford Flatheads use and are easy to find. I'd still read up on it as there are a few other things involved besides swapping rear axles and transmissions. I'm thinking that the Spring seats that Cl***ic parts sells for Chevy AD pickups will work on the cars and locate the axle in the right spot and at the right height off the springs. They are also 1-3/4 wide the same as the 50 Car springs.