On a PV, unfortunately, it involves cutting the tranmission tunnel out and crafting something a little roomier - it's too narrow aft of the current transmission to squeeze in an OD unit (which is actually wider than the transmission is). Although I have heard that the early 'D' type OD units are a little smaller, and can sometimes be fitted into a PV with just some bending/bashing/etc of the tunnel.

On my car I cut out the entire tunnel from midway around the shifter hole all the way back to right in front of the seat spar. I used a 122 tunnel for the main part (because I know it was roomy enough) ut found that the floorboards are lower in a PV (in relationship to the drivetrain) so I had to add an inch or so of new metal to the bottom edge. All in all I used 4 angle grinder wheels cutting a huge hole in the car, and a spool of MIG wire welding it back together. Not sure I would have done it to a nicer car, but a lot of what I replaced was rotten on my car anyhow.

Other than that major consideration, the only non-standard task was to get the driveshaft shortened. I took mine by a drivetrain shop and I think they charged me $70 to shorten and balance it. I got lucky in the output flange lottery when I got my M41, it just happened to come with the correct flange to mate up to the PV's driveshaft. I think there are 3 sizes, it can sometimes be difficult to get the right kind.

Love the OD, it was practically undriveable on the highway before (unless you didn't try to keep up with the 75 - 80 mph flow). The engine was just howling along at high rpms, sounded a bit abusive really. Now you slip it into OD and it seems at home. Especially if you've added some HP to your PV (mine probably has 170 - 180 hp, not much standard left on my motor) the OD will really let the car stretch its legs.

As for finding a mechanic that would do the above sorts of things to your car? Not a clue, but I can see how they'd be leery of such a project. It's beyond mere parts swapping (which, unfortunately, is all many mechanics are capable of). And the hours might add up to more than you think the end result is worth.

Another alternative to consider might be a change to the rear end axle ratio. It's a Dana/Spicer 27 unit, also used by various domestic cars of similar vintage. I think it might be possible to source a lower (numerically) ratio gear set for it. Trade off some acceleration for some top end cruising serenity.
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'63 PV544 rat rod, '93 Classic 245 + turbo