All you need to do is change the bold text information here:

[EFFECTS]
wake=fx_wake
water=fx_spray
dirt=fx_tchdrt
concrete=fx_sparks
touchdown=fx_tchdwn_s, 1
startup=cs737_engstart

In the 707-300, it is touchdown=cs707_tchdwn, 1
so if you prefer the 707-300 touchdown smoke over the default touchdown smoke used in the 737-200, you would need to change it like this:

[EFFECTS]
wake=fx_wake
water=fx_spray
dirt=fx_tchdrt
concrete=fx_sparks
touchdown=cs707_tchdwn, 1
startup=cs737_engstart

Just make sure that you backup the original aircraft.cfg before you make the changes. ;)