I took .030" off my 630 head, cleaned the ports/valve pockets, opened and re-worked the seat areas and re-shaped and polished the valves.. Blocked the "Heater Cavity" in the intake/exhaust area too keep heat and carbon debris out..
I can tell you, if you intend to use 87 Gas, you should be at about the limit, with your Gas pistons (for actual Farming)... Mine has been on my 4x16" semi-mount F-145 plow and handles it well, but IS a full load.. Needed 250# on the front, loaded rears, with 3 wts on each rear wheel..that is with new 15.5x38 rears.. 2nd gear all day... The power the 620/630 should put out is very impressive, without changing much. Ron.