I agree that dyno testing the motor would be best....... But....... There is no denying that the same motor tested on 10 different dynos would likely have at least a couple where the numbers were pretty far off from the others.
Just think of all the dyno numbers the average converter shop sees.
How many of those are from "happy" dynos?
I still say it's worthwhile to have it done, but I would pick a shop that has a pretty good track record with cars running close to what the dyno numbers say it should run.

All that being said......knowing what I do about your combo and what youre doing with it..... In the vein of "you gotta start somewhere"...... I would want a pretty tight 8" converter.
Around 5200 flash stall off the foot brake, and solidly under 10% slip out the back.