Yup, both the Weaver 63A (6.125" long) and the 63B (5.450" long) are flat bottomed one piece bases designed for the Marlin 336 or you could use the # 58 one piece base which is 4.624" long and .283" thick with a hole spacing of 3.275". If you go with two piece bases, here's some specs for you: #28 is .239" thick with a hole spacing of .504", #29 is.292" thick with a hole spacing of .504, #39 is .165" thick with a hole spacing of .504"...all the above listed one piece bases are .812" long and are flat bottomed as well.
Hope this info helps you a little...