Since clutch appears to work for forwards and with clutch depressed still won't reverse, than as your mechanic says, it's further down the drive line, probably trans. He's probably already diagnosed properly.
If not to your satisfaction, then need to further isolate problem. Something is likely broken in the driveline, that won't let it turn backward. As O_D says, you jack up rear and check brakes*, etc. Then disconnect the drive shaft and rule out if it's in the differential or in trans.
Any telltales? does it make grinding or other noise?

I doubt it's just the linkage, per se, even if that's messed up, it should roll backward with the clutch depressed regardless of what gear it's in, unless something peculiar to this year, like a reverse lock out system? I'm not familiar with this particular model. Generally, you only find those on shift mechanisms where it just won't allow shifter to go into reverse gear, but it still should roll back with clutch depressed. If clutch was frozen and not disengaging as O_D mentioned, you'd have problems switching gears, aka power shifting, and jumping when starter engaged.
* - Remotely, I could see a worn or frozen brake or ebrake, that only fully engages in the reverse direction, but that's doubtful. But there'd be a wheel noise or brake heat up going forward. Easy to check that by pulling rear wheels as indicated and testing the ebrake operation.

How about using the quarter places? I used to use them in snow/salt country when was too cold to drag out the hose at home. You get hot soapy water and filtered and treated rinse water.
Salt and now calcium/magnesium brine is very difficult to remove thoroughly, so I doubt your getting it all at the quickie car wash and need to follow up at home. Flush the fender wells and frame.

R.L. U B Stylin!

We all try to do everything, but sometimes it's best to get help/farm out. Repair by internet is tempting failure.

One for the museum/archive people. Not many left that would have been around to know...

A troll.
He could have asked the owner????
People fall for it all the time.

I guess the collector car hobby illusion of doing what it took a whole factory to do by one person continues.

WOW, thanks Howard. Just goes to show there's nothing new under the sun. I'd like to know how much he actually achieved HP/weight?
I guess if you're truly old school hot rod, you start with parts off something else. Fascinating these machinist geniuses, they can do anything. I visited with a commercial shop owner guy looking to buy his house, and he made a 4 gang rotary tow behind mower with V8 for his on lot golf "course". Envy of the neighborhood when you pull out that bad boy for your 0.25 acre burb lawn. I'd put one of those big fans on it so I'd blow the clippings on the neighbors yard! Everyone else goes to TSC, HT, or HF for Chinese "take out".
I was thinking engineer it, then CNC on a billet, but you have to have good dimensions of what you got. I'd rather just give $30K to Falconer for that alum turbo V12, but I'm too cheap. This whole indecisive motor issue has stopped my P project cold, not to mention the Cad taking precedence. The longer it takes the more I think things. That's half the fun...or is it agony or like fine wine?
I'd use my 288 for a stationary motor, but I have nothing to run with it. Otherwise, it's a garage space eater. As noted years ago, it's not worth much by itself and too big to put on a shelf, too good to junk.
Something to think about to anyone contemplating modifying a post war L8 based bathtub. Pick your motor first. If it came without a motor, I'd been better off.

Has anyone built or even contemplated a custom overhead valve head for a L eight? They made aluminum finned flat heads. They "routinely" make billet aluminum heads for V8 racing applications, and it would only be a bigger hunk of metal and machine job to make one in line. There would have to be some manipulation for an overhead cam but might solve the problem of adding mucho boost to a flat head. not sure but possible one could rework block sealing and make a stronger flat head to do the same.
The HP/weight gain would have to justify the $$. You start off very low, 130/800, or so. It's more like 0.5 min. in SBC land. Probably could light weight the block a bit. But should torq up a storm. Might just blow the old rings out of their grooves too.
It came in a dream...well, maybe a nightmare?
Tossed it on the wall for pro or cons?

I agree with OD, to a point. Shed light, not doubt. If your car has been protected and cared for "properly", it's far more likely to not suffer the ills of age. Proper storage, and a twice yearly detailing, goes a long way for paint as well. But what is "proper"? A term that's condition based and degree of fit for use dependent. Proper has a result. Improper does too, just not desired. Best practice applies generally.

What we refer to as "Pot metal" aka, zinc die cast, is zinc alloy in various formulas, so it's not as stable as 316 stainless steel. Colloquially, these terms have many meanings, "white metal" can also refer to tin/lead bases not used in these applications. Some die cast is lead based. I'm not referring to those.
The alloy composition, it's inclusions, surface treatment, and what it's subjected to, will in large part determine the stability of the part. One has no control over these, except care and storage conditions after acquiring. It could be great or already corroded inside or out. But corrosion requires things, oxygen and electron path, to proceed, and those can be interrupted.

Note the military test requirements in the two mil specs, C-11796C and 81309E for the VCI products. Much more severe than your garage.

PS: the bezel part may also be able to be made by machining, engraving, and followed by plating. I'd suggest to take it off, take digital photos, and make an impression now, so it can be duplicated later.