LTB is fine for 14' boats 900cfs and up, It is much different below 800cfs, but really just a couple really tight spots to make your day difficult, and a whole lot of rock dodging.
10-12' lightly loaded rafts are the best for low flow Rio. (R3-4) it is commercially run down to 500-ish. powerline and rock garden are the bad spots.
UTB is generally not run in rafts, except for adventurous dedicated rafters. not to dissuade you if thats your liking, but hiking boat and gear down the gorge into the Rio, plus Class sieved V is more than most want in a day. Its low volume,creeky up there, with 1/2 to 2/3 the flow downstream,as mania stated.
R2-R3 Super Puma is the recommended boat from what ive heard. class V kayakers love it early season.
LTB & Pilar Race course is so much easier logistically and safety wise, it is a better recommendation, and easy to do laps on. and there is good camping in the BLM state park in between, (look up Arroyo Hondo, Petaca, lone juniper, Taos junction bridge campgrounds. you shouldnt have to make reservations early season) with a float section to SUP or take family on.
American Whitewater - 5. John Dunn Bridge to Taos Junction Bridge (Lower Taos Box) , Rio Grande New Mexico, US