One other thing, RTV will not stick to a oily surface. You must use a brake clean type cleaner to really dry out the mating surfaces or oil will leach out under the RTV.

Main bearing caps that use dowel pins to locate them were used up to 1955 on 235's, after that, 1956 and later, they used some other method to locate the main caps with the crank. So if your engine has dowel pins locating the main caps, it can not be a '58-'62 235. A 261 is different so you could be dealing with one of those.

RTV on the back side of the rubber seal helps hold the steady in the block. Getting sealer on the back side only while the crank is in place is going to be very difficult.
If you can get a rear main rope seal from Best Gasket Company, you won't have any leaks and you will not have to pack the seal. They use a very good rope material thats pretty easy to install and could be done with crank in the block, but lowered (all the main caps off). I would not try it with only the rear cap off.

Joe

1937 Chevrolet Inline-powered Streetrod