I have occasionally gotten advice on building a performance b20 from a gentleman who races a b20 powered car. I have reason to require the absolute best possible oiling (it's a long story just why...). He recommended using a certain style- {I think 76-79.5} of *b21* oil pump body with the lower casting of the b20 pump. Has anyone done this? If so, why?- are the pump "gears" taller or of larger diameter? I live in an isolated area, so don't need the hassle and expense of special ordering several non returnable pumps to try to piece something together... Any inlet/outlet point that a guy can/should reshape to improve flow through the pump? Any advice apprieciated...

The advantage is that the B21 ( and B23 ) pumps use a plunger style relief valve, that centers the spring and prevents wear on the side of the spring. B20 and B18 pumps used a ball which allowed the spring to wear on the recess in the aluminum housing ( silicon crystals in the aluminum I suspect - George will know for sure )and the springs sometimes snapped, after losing tension of course, and lowering oil pressure. As to the gears, they are the same as far as I can tell.
I've done mod several times. It also meant I could use the IPD pump springs which have not been available for the ball type relief valve for many years.

Is this Ian Rankin?
Anyway, since oil is not compressible, the only advantage of "porting" an oil pump is to
help prevent cavitation at extreme speeds and to reduce the power to drive the pump.
It won't make much difference in throughput.
I advise you not to go to very high oil pressures (certainly not over 100 psi) because
you can blow the seals out from around the line from the oil pump to the block.
It happened to me when I got overzealous and put both a high pressure relief valve
spring and a washer behind it. I was pegging the oil pressure gauge which went from zero to 80 psi
through an arc of about 270° and it was going all the way about 357° back to the peg.
I estimate maybe 130-135 psi or more. Anyway one day I couldn't get more than about 25 psi
and when I tore it down the seal had blown out around the line going into the block.
I actually ran the car for a year with max oil pressure of 25 psi and normal to fast driving.
Did NOT see any unusual wear after that, in fact when I put a standard B18 (smaller)
oil pump in it it routinely carried 65 psi at anything over about 2000 rpm.
I think in normal operation a great deal of the oil escapes through the relief valve and you
still have enough to lube the bearings well and pump plenty up to the rockers.
(sorta like the fuel pump on a D-jet - no way could you burn all the fuel pump pumps!!)

Yes you can use an early B21 pump on a B20 as you described it.