If the clutch and all associated parts are new and the trans is really old it might be wise to think about the trans input bearing and input shaft to main shaft needle bearings. If they are worn the input shaft can "tilt" causing the clutch disc to drag enough to get your attention. This will spin the input shaft and cluster shaft.

When the cluster is spinning with the clutch engaged in neutral and the pedal is depressed ,,,even the slightest drag will keep it spinning.,,,,,making reverse quite a grind. The other gears will be silent as the synchros stop the cluster and input shaft. Lighter gear oil will make the situation worse as the lighter oil has less effect on slowing the cluster when the clutch is disengaged.

Make sure all the bellhousing and trans bolts are tight as any "tilt' will cause the same effect.

I have had heavy duty clutches bend the linkage making it hard to release the clutch.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

It's a good thing we don't discuss Toyotas on this board, or I'd have to tell the story of a coworker's Toyota van (the 'box on wheels' type from the mid '80s) with the hydraulic clutch that grew less and less effective. He thought he had a dying clutch master or slave cylinder. Nope, one day he walked in and dropped his clutch fork, bent at a nice 90° angle, on my desk.

(Oops...)

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一期一会
Too many people who were born on third base actually believe they've hit a triple.