Changing speedo drive gear from 8 to 7 tooth?

Was considering a gear change so was also considering a kit to calibrate the speedo correctly. Problem is, can this gear swap be done with the transmission in the car? Level of difficulty?? Job better left for tranny shop???

83-95 Mustang Speedometer Gear Kit For T5 With 3.73 Rear Gears by Ford at LRS - Same Day Shipping!

When using a speedcal, do you leave the stock driven speedo gear in place? Would I have to remove my current 23 tooth and reinstall the stock 18 (2.73 stock ratio) tooth speedo driven gear and then install the speedcal??

Not worried about the competent mechanic......justifying options of cost.

Option 1 - Speedcal is about $100 and not very difficult to install.

Option 2 - $25 - Yellow 7-tooth drive gear/21 tooth red driven requires tail end of transmission to be removed...which according to other things I've read......requires a removal of the shifter, cross member, and a transmission jack to hold tranny in place and to tilt it enough to allow the tail end to clear. Obviously with a gear change the driveshaft is getting removed anyways, so that point is moot.

Option 2 sounds like a lot more work and cost of labour to get it installed....not to mention the necessity of a lift to do it.

I'm not ruling out either option at this point until I can get some costs figured out.

Does the speecal require the driven gear be changed? I currently run a 23 tooth to match my 3.55s..