74 620 dirt track rear gear options

[circletrack22]

Hello all, newbie here, great site!  just bought a truck set up to run dirt circle track,  so that said, many parts are or may be different than body year.   Plus ,.no ID plates on truck .so here's the details and the dilemma.. I know it has a L18 and a 4 spd, we run a 1/4 mile dirt circle track,   here's the issue after the first few races, quickly realized truck was build for smaller track maybe 1/8 mile?  pulling out of the corner it is revved out by mid backstretch, already done the basic's advanced timing, etc

just looking for some help to get the truck to pull a little further down the straightaway with out throwing a ton of $$ at the motor as they do not allow in the rules 

Looking to find out

A- what rear end gears came stock? how can I ID them? 

B- can I run 720 gears for more options as the 620's are harder to find in the yards

 

I may have access to an 83 Datsun diesel rearend... wondering if that would bolt in and if the ratio would be different..

Been reading a lot of great things on here and I'm new to datsuns so I appreciate any knowledge anyone passes on!

 

Thanks!

[Guest Rick-rat]

What gear are you using in the tranny when racing?

[circletrack22]

2nd gear ...

[Guest Rick-rat]

3rd gear with a 4.37  rear will give you a 5.71 final, a 4.88 will give you a 6.38 final the 4.88 would probably work the best, If you have  a 4.37 rear gear now your final  is 9.07 in 2nd gear. any H190 pumpkin will work as long as the axle splines are the same