There should be room to spare if you are using the small distributor and short waterpump and brackets. If not, you have created your own problem but one answer to your question is 3/8" is enough clearence with a rigid fan, and marginal for a flex fan. Less than that and fan belt changes are a challenge. With close clearence good motor mounts are a must. Short pump setups were used on cars through 1968 and trucks through 1972. All the after market suppliers offer them because most of the earlier cars, 20's-40's, must have them for clearence.

Evan