MG Sports Cars

Are the mile markers on the highway accurate enough to calibrate it. Better way to have a measured 2 mile stretch (that's what's needed to calibrate the speedo).

Probably, but a gps is better.
Most speedo's will show an impulse count when you're doing the calibration run.
Write down the number when you reach the end of your calibration.
Now drive the car and compare the indicated speed to the gps.
It will be close but probably not right on the money.
Rerun your calibration run and either end it early or later depending on whether you are too slow or too fast.
The nice part is now that you have a number to compare you can do the run in your driveway with the rear up on axle stands.
Once you are dialed in, copy down your final number. Lots of speedo's lose their memory from time to time and this number will be easy to re-enter.
Cheers
Fred

P.S. have you gotten your carb and timing dialed in yet?
That kind of got forgotten in the miss fire shuffle.

I calibrated mine using one marked mile. On a trip, I checked it for 218 miles (using mile markers) and it was off by less than 0.5 miles over the distance. So, it seems using a one mile stretch for calibratiion can be very accurate.