You did the math right.
Now to size the injectors:
For a boosted engine, use a BSFC of 0.55 lbs of fuel/HP-hr
So for 250 HP you need 137.5 lbs of fuel per hour
Spread over 6 injectors that would be 22.9 lbs/hr each.
since you want to work an injector at 90% duty cycle of less that would be a rating of 25.5 at your actual pressure drop
Injectors are typically rated at 39 PSI but if you use the stock fuel pressure regulator for a 300 that goes up to 55 PSI or more. However when you increase the manifold pressure with a turbo, you have to lower the injector pressure drop by a like amount. Since you're looking at 10 PSIG boost, that would mean the actual pressure drop becomes 45 PSI. The rating of an injector increases or decreases as the square root of the increase or decreases in pressure drop. So at 45 PSI your injectors would flow (45/39)^1/2 of their rating or about 107.4%
If you need an actual flow of 25.5, you would have to have a rated flow at 39 PSI of 23.7 lbs/hr.
It looks like 24 lb. injectors will just barely do the job with that 10% margin of safety that restricting the duty cycle gives you. If you back the boost off to 8 PSI, 24s will work like a champ.
You do not want to go bigger than you have to on injectors because they can run overly rich when you're off boost.
You need a fuel pump which can supply at least 10% more fuel than the injectors demand or about 151 lbs per hour at 6 lbs per gallon, that's 25.2 gallons/hr or 95 litres/hr. The stock pump for a 96 is rated at 95 LPH at 55 PSI so you might just barely squeak by if you were building a a 96. Problem is that Ford has used several completely different fuel pump systems over the years so to work out the pumps, I'd need to know which system you have
As for sizing the turbo, you need to learn how to read a turbo map to pick the one for your application. I'll leave that part of the project to you