well i have a 510 as well and did an sr20 swap on it. and would like to auto cross with it possibly. as for my 620. i drove by a few times and was digging the truck and saw potential in it. I have no seen any 73 620 as far as around town. the work that i have done so far is i have bagged it and four linked the rear. I also made custom mounts and installed a high volt chevy alt. the reason for doing this was to help accommodate the draw from the pumps.(the original alt didnt cut it) . i would like to know what other manual trannys will fit behind the l16? engine work... not sure, i would really like to find a manifold to fit the R1 carbs (which i have already). and i dont know yet?