My first car, a '75 corona, had a blown output seal on tranny, so we drilled a hole through the floor pan, glued a bit of garden hose into the filler hole on side of trannie, and top it up while you drive. only used about a litre an hour.
a series run a 180mm wide clutch disc, the l series run a 200mm. the 63a spline is the same as the l series, so you need to either have your flywheel machined and redrilled to accept a 200mm disc or get a clutch joint to make you a 180mm disc with the same spline as the l series 200mm one.
the output spline at the back is the same as l series too, so a new shaft will have to be made, my shaft was already fitted with 180B unis, so I just knocked the original front yoke off and grabbed a 180B one from the wreckers and whacked that on and got it balanced. You might need to get something made.
Depending on whether your coupe started life as a manual or automatic, you may need to rework the tunnel slightly to make it fit in, especially around the reverse light switch on the box and the original crossmember mounts in the tunnel. A new crossmember will also have to be made.
I dont think that just a rebuild would throw the numbers out that much, you'd have to change the gears inside to some very different ones to get the inaccuracy your talking about.
You'r not by any chance running REALLY low profile tyres on your 13's??
running the non-standard tyres may be defectable depending on the state law which applies to you, but the accuracy of the speedo cant really be determined by the police on the side of the road without a dyno.
Dont forget that diff ratios and gearbox ratios also affect the speedo reading.