iRule using TCP::collect and TCP::payload to fix blanks (spaces) in HTTP request

The final(ish) version is this:

when CLIENT_ACCEPTED {
log local0. "-"
    TCP::collect
}

when CLIENT_DATA {
log local0. "-"
    if { [TCP::payload 28] contains "GET /?engine=google&s_kwcid=" } { 
log local0. "Google Keyword Match"
        set mymatch [findstr [TCP::payload] "GET" 0 "\r\n"]
             set mybal [findstr [TCP::payload] "\r\n" 0]  
             set myfields [split $mymatch " "] 
             if { [llength $myfields] > 3 } { 
log local0. "Found Blanks in Request"
                set mynewreq "[concat [lindex $myfields 0] [join [lrange $myfields 1 end-1] "+"] [lindex $myfields end]]"
log local0. "Request Fixed : $mynewreq"
        } else { 
log local0. "No Blanks in Request"
                set mynewreq $mymatch 
        }

             TCP::payload replace 0 [TCP::payload length] ""
             TCP::payload replace 0 0 "[concat $mynewreq $mybal]\r\n\r\n"
             TCP::payload replace 0 0 "$mynewreq$mybal\r\n\r\n"
log local0. "TCP::payload -> [TCP::payload]"

       }    
          TCP::release
          TCP::collect

    }

The trick, as nitass identified, is concatenating the resulting strings w/out squeezing white-space needed to be compliant with the protocol. I ended up avoiding concat altogether, and it works, but I'm afraid of it working on a fluke and possibly breaking after an upgrade. Is it valid syntax to build the payload like this?:

TCP::payload replace 0 0 "$mynewreq$mybal\r\n\r\n"

If this is supported, I'm fine with it. If there is a better way to do it, I'm open to hear it. How have others solved this problem?

Thanks for any insights.

Mike