73 400 should be fine...I have a 72 400 in mine...you might just want to switch out heads..Im` not sure which are on a 73 , but 72 had 7J2`s, not very peppy...I got myself some 75 5Cs, milled down to 88 cc..
as long as you have the large valves and 86-90+_ CC`s you will get a strong engine for street use...should give yu a 9.2+_ CR , you could run regular...