If it was me, I would just go with a 289 or a 302 motor, they are the easiest to install I find. The big blocks would require you to bend or cut your shock towers since it needs more room. I have heard though that the 351C is a very good motor and can take a lot before it blows, but I would just stick with the 5.0L, I think that is your best bet. I have a 5.0 in my 67 Mustang and another in a 71 Torino, and they give me no problems at all. Also, remember that prices for parts on big blocks are more expensive then small blocks...say you need a new water pump or want better valve covers, you will pay more for the 390+ motors then you would for small blocks.
1981 DMC DeLorean
1966 Thunderbird 390 FE HI PERF T-L (Red)
1966 Thunderbird 390 T-L (Black)
1950 Buick Special
1997 Subaru Impreza AWD