[c-nsp] Manually set WS-X6148-GE-TX MTU size (1500, 1518)

[falz]

Specs on WS-X6148-GE-TX say there is a maximum MTU of 1518:

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/product_data_sheet0900aecd8017376e_ps4835_Products_Data_Sheet.html

However, on a 6500 running SXH, it will not let me use the mtu command
to adjust. I am trying to up the MTU for MPLS. Any way to do this
manually or is this something that's supported in hardware and
automatically upped slightly if a port were a trunk port, for example?

Trying to avoid purchasing WS-X6516-GE-TX or WS-X6748-GE-TX if possible.