51 is good, the 49 to 53's are most refined of the bunch [cl
plus you alread have the "big" crank 4in stroke the rest were 3.75in the only differences were in bore on the earlier fords (pre 49)
Pick up a copy of 'FORD FLATHEAD V-8 BUILDERS HANDBOOK 1932-1953' by Frank Otto lots great how to stuff and every good or bad thing about flattie ford v8's you ever wanted to know
specs for all years bearing info torque spec etc etc , common failure points and areas where the blocks like to crack what to look for when buyin a used flatie..... the list goes on!!!!
my local public library had a copy on the shelf
then google what replacement or speed parts cost, then decide if you REALLY want a flathead. i'm leaning toward a late model swap but i may go thru the flattie and see if she's saveable after I get my 50 sedan on the road