the 87GTL is the base station version of the 29GTL, which is very, very similar to the 29LTD.
so, with a little comparing of the schematics, most, if not all mods that are for the 29LTD could be done to the 87GTL.
that being said, there are minor differences, and those need to be worked out before attempting the mods.
that is why we cant just give you a list of mods for that radio.
i can tell you that VR6 is your modulation control, and that L17 and L13 are the coils to peak for increased modulated power out.
(that means peak them for max watts with modulation applied, NOT max deadkey)
here is a link to the schematic of the 87GTL:
http://www.cbtricks.com/radios/cobra/87gtl/graphics/cobra_87gtl_sch.pdf
here is a link to the schematic of the 29LTD:
http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/cobra_29_ltd_classic_sch.pdf
when you find a mod for the 29LTD (they are all over the internet), just compare the schematics to see what changes you need to make to make them work with the 87GTL.
good luck,
LC