727 hard shift from park to reverse

In reverse, the front pump output is not regulated besides the hi-pressure relief valve.
To make reverse, the trans uses the Hi-drum and the L/R band.
As mentioned, the Hi-drum uses a wavy spring to cushion engagement.
The L/R servo uses a spring to slow it down.
Now it just so happens that the Accumulator spring is about the same size and fits in there very nicely......... but it is a far stiffer spring. If those two are swapped , Bam! as soon as the L/R servo sees pressure it instantly slams the band.

To make First gear, the trans uses two methods;
If you put it into Drive, the trans will engage the Forward clutch and use the rear Sprag to make sure the output shaft turns in the correct direction. The L/R band is not used.
In Manual Low. the L/R band is used again to anchor the business.
You can use this oddity to help diagnose the servo-spring.
If it shifts softly into Drive, then all is well. But
if it bangs into BOTH manual Low and Reverse, I would suspect the servo spring. Especially if grabbing Second is a lil mushy