From my own humble understanding of the tax laws
and experience in the MSTR-SZA merger this year:

Since you started with 6 lots, each with their own
cost basis, you will still have 6 lots, each with the same cost basis as the original lots.
Just use the exchange ratio to determine the number of shares in each new lot. If you receive fractional shares in cash instead of stock, they come from the first lot (based on FIFO).

Here is my experience from the MSTR-SZA merger:

a) 160 shares MSTR at 26 1/2 + 12 commisions + 501.47 from a previous wash-sale-rule = 4753.47 cost basis

b) 130 shares MSTR at 40 1/8 + 12 commisions = 5228.25 cost basis

1 MSTR = 0.85 SZA merger-deal exchange ratio

a) 160 MSTR = 136 SZA with cost basis = 4753.47
b) 130 MSTR = 110.5 SZA with cost basis = 5228.25

Total = 246.5 SZA shares

but there is a 0.5 SZA fractional share that was issued as cash not stock, so it is a realized gain from the first lot (FIFO), even though the second lot is what generated the fractional share.

a) 136 SZA == 135.5 SZA with cost basis = (135.5/136)*4753.47 = 4735.99
plus 0.5 SZA with cost basis = (0.5/136)*4753.47 = 17.48

b) 110.5 SZA with cost basis = 5228.25

cash received for 0.5 SZA = 29.13,
so capital gain = 29.13 - 17.48 = 11.65.

Whether or not the capital gain is a short or long term gain depends on how long you held the first lot (since that is where the fractional share is taken from).

Also, since you can't sell fractional shares, if I wanted to sell 150 SZA on a FIFO basis the first 135.5 would be from the first lot, the last 14.5 would be from the second lot. (Therefore, some lot mixing is inevitable.)

You should definitely check the IRS web site to find the publications related to this matter (I think it is Tax-free exchanges of property &/or Basis of property)
That should point you in the right direction in case I have misinterpreted the tax laws.

Good Luck!