Haven't tried them myself but others here may have ? I used the Boogie STRs in my MP1 (most likely re-badged JJs but not sure which version, short plates though IIRC).
Longplates are a little more prone to microphonics compared to short plates but I don't think that would be a big issue for your MP1 (depending where you mount it e.g. on top of loud speaker box = more vibration). MJMP advises not to put long plates in 3TM (but that's super high gain (3 tubes...)) and makes sense.
Personally, I'd try the Mullard long plates (http://www.dougstubes.com/preamp-tubes/12ax7-ecc83-7025/mullard-12ax7.html), I use them in both my MP2s, great tube, very 3D, very smooth. They also do a short plate model Mullard (http://www.dougstubes.com/preamp-tubes/12ax7-ecc83-7025/mullard-cv4004-12ax7.html) that one of the guys here tried in his MP1 (and liked allot). Also from the old Depot board, a member tried them (also MP1, long plates), a he loved them.
Anyway let us know how you go, I'm sure the JJ 803s will sound fine.