Can I replace my furnace motor's startup capacitor with one matching its specifications?
You can purchase them according to the specification.
5uF 370VAC for one, and 7.5uF 370VAC for the second.
Your repair man was there to fix something - they are working .. sounds like a suspicious charge to me. It is very unusual for a repair man to break out their capacitor checker and check UF of the Capacitors. The are +- 10%  (so 4.5UF to 5.5UF ) is fine. On the 7.5 UF ( 6.75 to 8.25 UF is acceptable ranges) a meter  that has a capacitance checker on it is what is needed to determine the capacitance - generally cheapo's do not have this function - although I have a cheapo (not really but cheaper than a fluke) I also have a fluke that measures this.
What reasoning is there that these are out of tolerance ?
This is the more relevant question
Is the unit experiencing any problems - what issues does it have .. ??
Because if they are working - what problem is there that he/she stated they were out of tolerance ?
Did he/she physically measure them and visually show you on his meter the reading?
If you are not experiencing any other problem - I would be suspect of the statement they are out of tolerance, especially if they did not show me the reading.
Typically you can buy these for $5 to $10 a piece - the repair guy will charge you $35 each or more plus labor. Another $150 - $200 . Ka' Ching and many people will be suckered to pay it.