That used to be run by Peter Englezos
not sure if company exists anymore, he used to do all the race boxes and the close ratio highway cruising economy gears
its a trans-axle so its a box and diff build in one i.e you are paying for 2 jobs....
i used to just get another box and swap the car or van appropriate, nose cone on. some you win some you lose i had a few boxes
you will want a swing axle box not IRS so avoid 1302 or 1303 or late type 3 boxes
to run a late box from say a 1500 in an early car you need the right nose cone and shift rod. for an oval that was from a from a split screen van, don't think you need to do that for a 61 anyway you already have an appropriate nose cone and shift rod on your old box, get the brass bush/seal replaced for the shift rod stops the oil coming out the front all over the mount.
so a good box with 61 appropriate axles and tubes, use the ones you have, i.e correct length and bolt pattern to fit between the spring plates already attached to the car, and the nose cone off you knackered box would work. be aware of spline and nut size differences for the hubs if you don't use your axles and tubes....no use having a box in and not being able to fit the brakes and wheels.....
keep in mind the bellhousing size.... 1200 1300 bellhousing won't take a 1600 flywheel. only an issue if you have 1600 flywheel and want to use an early box. get engine half in with no clutch on it, do up bolts evenly start it and the ring gear will cut the bell housing a bit bigger, welding gloves on, and waggle and shove the engine via exhaust and fan shroud until it fits

remove the rim of what you just cut with a grinder... its a bodge but it works. or get it done properly at a machine shop...
clutch fork issues the correct release bearing for the fork isn't necessarily the correct one for the type of clutch on your motor, diaphragm or the 3 finger type.
but they all work, will just need adjustment. curved face bearing sticks out a bit further than the flat early type
put a new clutch cable bowden tube on with the correct Bow to it, while you are at it. stop that clutch judder.
all of this is "I think"..... i'm out of date, its 15- 20 years since i worried about any of this stuff
have you thought about Subaru gears? subaru transaxle with a custom reversed ring and pinion so it can be used in a bug.... gets you a 5 speed box
i gave up messing eventually and had the rear end of a bay window van welded into my splitty so i could run 1303s box and gearing think i had 1303 mexico/brazil box
Harry Harpics in south end used to be the guru for fitting IRS to cars that were never supposed to have it. take out torsion bars weld on brackets bolt up arms and you can run CV jointed IRS rear end
handles like a golf..... well not really.. but better.
Paul, i think, still runs the business or maybe his son. Paul is a good guy..
Would certainly be worth a call
Dave