Difficulty deciphering the RSA algorithm $d = 27, e = 3, n = 40$

For encryption, I have the letter T converted to 084 by the ASCII table, which encrypts to $24$ by $84^3\bmod 40$, for example. 

To decrypt, I believe it would be $24^{27}\bmod40$ however this gives me 4.4342649e+38 which clearly does not fit into my ASCII table.  

How does one decrypt using the RSA algorithm in relation to ASCII?

Further question: Is ASCII converter the best way to convert plaintext to numerical value?

EDIT: this is purely for a high school maths project; thank you for all the answers
You have two problems. The first is that you tried encrypting a value which is too large. If you are doing all your math mod n you have no hope of ever getting a result larger than n. 

The second is in tthe implementation. You seem to be working in an environment which treats large numbers as floating points (JavaScript? ) and are not using efficient modular exponentiation.
https://en.m.wikipedia.org/wiki/Modular_exponentiation

24^27 is a fairly large number. It doesn't fit in a normal int or long variable. So you need to either have a special big integer to handle such large numbers. Or Preferably avoid them in the first place by applying the moduls in intermediate steps while calculating the power operation, preventing the nunbers you are handling from getting out of hand.(In full size implementations you need both).