Probably would be best to have the jumper at the radio end for several reasons. First, it's indoors (most likely) so weatherproofing won't be an issue. Second, since it's LMR-400 and quite stiff, a short jumper is probably best anyway to reduce the strain on the radio's antenna connection.
While it may be best to use a specialized connector for the joint (like a direct mating connector to what's already on the coax). Best case is your LMR-400 already has an male "N" connector and then you can order your jumper with the female "N", but if it's already got a PL-259 and your jumper does as well, just use a standard barrell connector and call it a day. If the extra .25 dB is that important, you need to run a new feed with the correct length and use something better than LMR-400 anyway.